Add "--force-reinstall" and a comment as to why to our "pip install" lines

2.7/Dockerfile

@@ -47,7 +47,12 @@ RUN set -ex \
 		&& wget -O /tmp/get-pip.py 'https://bootstrap.pypa.io/get-pip.py' \
 		&& python2 /tmp/get-pip.py "pip==$PYTHON_PIP_VERSION" \
 		&& rm /tmp/get-pip.py \
-	&& pip install --no-cache-dir --upgrade "pip==$PYTHON_PIP_VERSION" \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	&& pip install --no-cache-dir --upgrade --force-reinstall "pip==$PYTHON_PIP_VERSION" \
+# then we use "pip list" to ensure we don't have more than one pip version installed
+# https://github.com/docker-library/python/pull/100
 	&& [ "$(pip list |tac|tac| awk -F '[ ()]+' '$1 == "pip" { print $2; exit }')" = "$PYTHON_PIP_VERSION" ] \
 	\
 	&& find /usr/local -depth \

2.7/alpine/Dockerfile

@@ -64,7 +64,12 @@ RUN set -ex \
 		&& wget -O /tmp/get-pip.py 'https://bootstrap.pypa.io/get-pip.py' \
 		&& python2 /tmp/get-pip.py "pip==$PYTHON_PIP_VERSION" \
 		&& rm /tmp/get-pip.py \
-	&& pip install --no-cache-dir --upgrade "pip==$PYTHON_PIP_VERSION" \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	&& pip install --no-cache-dir --upgrade --force-reinstall "pip==$PYTHON_PIP_VERSION" \
+# then we use "pip list" to ensure we don't have more than one pip version installed
+# https://github.com/docker-library/python/pull/100
 	&& [ "$(pip list |tac|tac| awk -F '[ ()]+' '$1 == "pip" { print $2; exit }')" = "$PYTHON_PIP_VERSION" ] \
 	\
 	&& find /usr/local -depth \

2.7/slim/Dockerfile

@@ -60,7 +60,12 @@ RUN set -ex \
 		&& wget -O /tmp/get-pip.py 'https://bootstrap.pypa.io/get-pip.py' \
 		&& python2 /tmp/get-pip.py "pip==$PYTHON_PIP_VERSION" \
 		&& rm /tmp/get-pip.py \
-	&& pip install --no-cache-dir --upgrade "pip==$PYTHON_PIP_VERSION" \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	&& pip install --no-cache-dir --upgrade --force-reinstall "pip==$PYTHON_PIP_VERSION" \
+# then we use "pip list" to ensure we don't have more than one pip version installed
+# https://github.com/docker-library/python/pull/100
 	&& [ "$(pip list |tac|tac| awk -F '[ ()]+' '$1 == "pip" { print $2; exit }')" = "$PYTHON_PIP_VERSION" ] \
 	\
 	&& find /usr/local -depth \

2.7/wheezy/Dockerfile

@@ -47,7 +47,12 @@ RUN set -ex \
 		&& wget -O /tmp/get-pip.py 'https://bootstrap.pypa.io/get-pip.py' \
 		&& python2 /tmp/get-pip.py "pip==$PYTHON_PIP_VERSION" \
 		&& rm /tmp/get-pip.py \
-	&& pip install --no-cache-dir --upgrade "pip==$PYTHON_PIP_VERSION" \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	&& pip install --no-cache-dir --upgrade --force-reinstall "pip==$PYTHON_PIP_VERSION" \
+# then we use "pip list" to ensure we don't have more than one pip version installed
+# https://github.com/docker-library/python/pull/100
 	&& [ "$(pip list |tac|tac| awk -F '[ ()]+' '$1 == "pip" { print $2; exit }')" = "$PYTHON_PIP_VERSION" ] \
 	\
 	&& find /usr/local -depth \

3.3/Dockerfile

@@ -56,7 +56,12 @@ RUN set -ex \
 		&& python3 /tmp/get-pip.py "pip==$PYTHON_PIP_VERSION" \
 		&& rm /tmp/get-pip.py \
 	; fi \
-	&& pip3 install --no-cache-dir --upgrade "pip==$PYTHON_PIP_VERSION" \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	&& pip3 install --no-cache-dir --upgrade --force-reinstall "pip==$PYTHON_PIP_VERSION" \
+# then we use "pip list" to ensure we don't have more than one pip version installed
+# https://github.com/docker-library/python/pull/100
 	&& [ "$(pip list |tac|tac| awk -F '[ ()]+' '$1 == "pip" { print $2; exit }')" = "$PYTHON_PIP_VERSION" ] \
 	\
 	&& find /usr/local -depth \

3.3/alpine/Dockerfile

@@ -73,7 +73,12 @@ RUN set -ex \
 		&& python3 /tmp/get-pip.py "pip==$PYTHON_PIP_VERSION" \
 		&& rm /tmp/get-pip.py \
 	; fi \
-	&& pip3 install --no-cache-dir --upgrade "pip==$PYTHON_PIP_VERSION" \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	&& pip3 install --no-cache-dir --upgrade --force-reinstall "pip==$PYTHON_PIP_VERSION" \
+# then we use "pip list" to ensure we don't have more than one pip version installed
+# https://github.com/docker-library/python/pull/100
 	&& [ "$(pip list |tac|tac| awk -F '[ ()]+' '$1 == "pip" { print $2; exit }')" = "$PYTHON_PIP_VERSION" ] \
 	\
 	&& find /usr/local -depth \

3.3/slim/Dockerfile

@@ -69,7 +69,12 @@ RUN set -ex \
 		&& python3 /tmp/get-pip.py "pip==$PYTHON_PIP_VERSION" \
 		&& rm /tmp/get-pip.py \
 	; fi \
-	&& pip3 install --no-cache-dir --upgrade "pip==$PYTHON_PIP_VERSION" \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	&& pip3 install --no-cache-dir --upgrade --force-reinstall "pip==$PYTHON_PIP_VERSION" \
+# then we use "pip list" to ensure we don't have more than one pip version installed
+# https://github.com/docker-library/python/pull/100
 	&& [ "$(pip list |tac|tac| awk -F '[ ()]+' '$1 == "pip" { print $2; exit }')" = "$PYTHON_PIP_VERSION" ] \
 	\
 	&& find /usr/local -depth \

3.3/wheezy/Dockerfile

@@ -56,7 +56,12 @@ RUN set -ex \
 		&& python3 /tmp/get-pip.py "pip==$PYTHON_PIP_VERSION" \
 		&& rm /tmp/get-pip.py \
 	; fi \
-	&& pip3 install --no-cache-dir --upgrade "pip==$PYTHON_PIP_VERSION" \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	&& pip3 install --no-cache-dir --upgrade --force-reinstall "pip==$PYTHON_PIP_VERSION" \
+# then we use "pip list" to ensure we don't have more than one pip version installed
+# https://github.com/docker-library/python/pull/100
 	&& [ "$(pip list |tac|tac| awk -F '[ ()]+' '$1 == "pip" { print $2; exit }')" = "$PYTHON_PIP_VERSION" ] \
 	\
 	&& find /usr/local -depth \

3.4/Dockerfile

@@ -56,7 +56,12 @@ RUN set -ex \
 		&& python3 /tmp/get-pip.py "pip==$PYTHON_PIP_VERSION" \
 		&& rm /tmp/get-pip.py \
 	; fi \
-	&& pip3 install --no-cache-dir --upgrade "pip==$PYTHON_PIP_VERSION" \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	&& pip3 install --no-cache-dir --upgrade --force-reinstall "pip==$PYTHON_PIP_VERSION" \
+# then we use "pip list" to ensure we don't have more than one pip version installed
+# https://github.com/docker-library/python/pull/100
 	&& [ "$(pip list |tac|tac| awk -F '[ ()]+' '$1 == "pip" { print $2; exit }')" = "$PYTHON_PIP_VERSION" ] \
 	\
 	&& find /usr/local -depth \

3.4/alpine/Dockerfile

@@ -73,7 +73,12 @@ RUN set -ex \
 		&& python3 /tmp/get-pip.py "pip==$PYTHON_PIP_VERSION" \
 		&& rm /tmp/get-pip.py \
 	; fi \
-	&& pip3 install --no-cache-dir --upgrade "pip==$PYTHON_PIP_VERSION" \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	&& pip3 install --no-cache-dir --upgrade --force-reinstall "pip==$PYTHON_PIP_VERSION" \
+# then we use "pip list" to ensure we don't have more than one pip version installed
+# https://github.com/docker-library/python/pull/100
 	&& [ "$(pip list |tac|tac| awk -F '[ ()]+' '$1 == "pip" { print $2; exit }')" = "$PYTHON_PIP_VERSION" ] \
 	\
 	&& find /usr/local -depth \

3.4/slim/Dockerfile

@@ -69,7 +69,12 @@ RUN set -ex \
 		&& python3 /tmp/get-pip.py "pip==$PYTHON_PIP_VERSION" \
 		&& rm /tmp/get-pip.py \
 	; fi \
-	&& pip3 install --no-cache-dir --upgrade "pip==$PYTHON_PIP_VERSION" \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	&& pip3 install --no-cache-dir --upgrade --force-reinstall "pip==$PYTHON_PIP_VERSION" \
+# then we use "pip list" to ensure we don't have more than one pip version installed
+# https://github.com/docker-library/python/pull/100
 	&& [ "$(pip list |tac|tac| awk -F '[ ()]+' '$1 == "pip" { print $2; exit }')" = "$PYTHON_PIP_VERSION" ] \
 	\
 	&& find /usr/local -depth \

3.4/wheezy/Dockerfile

@@ -56,7 +56,12 @@ RUN set -ex \
 		&& python3 /tmp/get-pip.py "pip==$PYTHON_PIP_VERSION" \
 		&& rm /tmp/get-pip.py \
 	; fi \
-	&& pip3 install --no-cache-dir --upgrade "pip==$PYTHON_PIP_VERSION" \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	&& pip3 install --no-cache-dir --upgrade --force-reinstall "pip==$PYTHON_PIP_VERSION" \
+# then we use "pip list" to ensure we don't have more than one pip version installed
+# https://github.com/docker-library/python/pull/100
 	&& [ "$(pip list |tac|tac| awk -F '[ ()]+' '$1 == "pip" { print $2; exit }')" = "$PYTHON_PIP_VERSION" ] \
 	\
 	&& find /usr/local -depth \

3.5/Dockerfile

@@ -56,7 +56,12 @@ RUN set -ex \
 		&& python3 /tmp/get-pip.py "pip==$PYTHON_PIP_VERSION" \
 		&& rm /tmp/get-pip.py \
 	; fi \
-	&& pip3 install --no-cache-dir --upgrade "pip==$PYTHON_PIP_VERSION" \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	&& pip3 install --no-cache-dir --upgrade --force-reinstall "pip==$PYTHON_PIP_VERSION" \
+# then we use "pip list" to ensure we don't have more than one pip version installed
+# https://github.com/docker-library/python/pull/100
 	&& [ "$(pip list |tac|tac| awk -F '[ ()]+' '$1 == "pip" { print $2; exit }')" = "$PYTHON_PIP_VERSION" ] \
 	\
 	&& find /usr/local -depth \

3.5/alpine/Dockerfile

@@ -73,7 +73,12 @@ RUN set -ex \
 		&& python3 /tmp/get-pip.py "pip==$PYTHON_PIP_VERSION" \
 		&& rm /tmp/get-pip.py \
 	; fi \
-	&& pip3 install --no-cache-dir --upgrade "pip==$PYTHON_PIP_VERSION" \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	&& pip3 install --no-cache-dir --upgrade --force-reinstall "pip==$PYTHON_PIP_VERSION" \
+# then we use "pip list" to ensure we don't have more than one pip version installed
+# https://github.com/docker-library/python/pull/100
 	&& [ "$(pip list |tac|tac| awk -F '[ ()]+' '$1 == "pip" { print $2; exit }')" = "$PYTHON_PIP_VERSION" ] \
 	\
 	&& find /usr/local -depth \

3.5/slim/Dockerfile

@@ -69,7 +69,12 @@ RUN set -ex \
 		&& python3 /tmp/get-pip.py "pip==$PYTHON_PIP_VERSION" \
 		&& rm /tmp/get-pip.py \
 	; fi \
-	&& pip3 install --no-cache-dir --upgrade "pip==$PYTHON_PIP_VERSION" \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	&& pip3 install --no-cache-dir --upgrade --force-reinstall "pip==$PYTHON_PIP_VERSION" \
+# then we use "pip list" to ensure we don't have more than one pip version installed
+# https://github.com/docker-library/python/pull/100
 	&& [ "$(pip list |tac|tac| awk -F '[ ()]+' '$1 == "pip" { print $2; exit }')" = "$PYTHON_PIP_VERSION" ] \
 	\
 	&& find /usr/local -depth \

3.5/windows/windowsservercore/Dockerfile

@@ -42,7 +42,10 @@ RUN $url = ('https://www.python.org/ftp/python/{0}/python-{1}-amd64.exe' -f $env
 	\
 	$pipInstall = ('pip=={0}' -f $env:PYTHON_PIP_VERSION); \
 	Write-Host ('Installing {0} ...' -f $pipInstall); \
-	pip install --no-cache-dir --upgrade $pipInstall; \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	pip install --no-cache-dir --upgrade --force-reinstall $pipInstall; \
 	\
 	Write-Host 'Verifying pip install ...'; \
 	pip --version; \

3.6/Dockerfile

@@ -50,7 +50,12 @@ RUN set -ex \
 		&& python3 /tmp/get-pip.py "pip==$PYTHON_PIP_VERSION" \
 		&& rm /tmp/get-pip.py \
 	; fi \
-	&& pip3 install --no-cache-dir --upgrade "pip==$PYTHON_PIP_VERSION" \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	&& pip3 install --no-cache-dir --upgrade --force-reinstall "pip==$PYTHON_PIP_VERSION" \
+# then we use "pip list" to ensure we don't have more than one pip version installed
+# https://github.com/docker-library/python/pull/100
 	&& [ "$(pip list |tac|tac| awk -F '[ ()]+' '$1 == "pip" { print $2; exit }')" = "$PYTHON_PIP_VERSION" ] \
 	\
 	&& find /usr/local -depth \

3.6/alpine/Dockerfile

@@ -67,7 +67,12 @@ RUN set -ex \
 		&& python3 /tmp/get-pip.py "pip==$PYTHON_PIP_VERSION" \
 		&& rm /tmp/get-pip.py \
 	; fi \
-	&& pip3 install --no-cache-dir --upgrade "pip==$PYTHON_PIP_VERSION" \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	&& pip3 install --no-cache-dir --upgrade --force-reinstall "pip==$PYTHON_PIP_VERSION" \
+# then we use "pip list" to ensure we don't have more than one pip version installed
+# https://github.com/docker-library/python/pull/100
 	&& [ "$(pip list |tac|tac| awk -F '[ ()]+' '$1 == "pip" { print $2; exit }')" = "$PYTHON_PIP_VERSION" ] \
 	\
 	&& find /usr/local -depth \

3.6/slim/Dockerfile

@@ -63,7 +63,12 @@ RUN set -ex \
 		&& python3 /tmp/get-pip.py "pip==$PYTHON_PIP_VERSION" \
 		&& rm /tmp/get-pip.py \
 	; fi \
-	&& pip3 install --no-cache-dir --upgrade "pip==$PYTHON_PIP_VERSION" \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	&& pip3 install --no-cache-dir --upgrade --force-reinstall "pip==$PYTHON_PIP_VERSION" \
+# then we use "pip list" to ensure we don't have more than one pip version installed
+# https://github.com/docker-library/python/pull/100
 	&& [ "$(pip list |tac|tac| awk -F '[ ()]+' '$1 == "pip" { print $2; exit }')" = "$PYTHON_PIP_VERSION" ] \
 	\
 	&& find /usr/local -depth \

3.6/windows/windowsservercore/Dockerfile

@@ -36,7 +36,10 @@ RUN $url = ('https://www.python.org/ftp/python/{0}/python-{1}-amd64.exe' -f $env
 	\
 	$pipInstall = ('pip=={0}' -f $env:PYTHON_PIP_VERSION); \
 	Write-Host ('Installing {0} ...' -f $pipInstall); \
-	pip install --no-cache-dir --upgrade $pipInstall; \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	pip install --no-cache-dir --upgrade --force-reinstall $pipInstall; \
 	\
 	Write-Host 'Verifying pip install ...'; \
 	pip --version; \

Dockerfile-alpine.template

@@ -67,7 +67,12 @@ RUN set -ex \
 		&& python3 /tmp/get-pip.py "pip==$PYTHON_PIP_VERSION" \
 		&& rm /tmp/get-pip.py \
 	; fi \
-	&& pip3 install --no-cache-dir --upgrade "pip==$PYTHON_PIP_VERSION" \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	&& pip3 install --no-cache-dir --upgrade --force-reinstall "pip==$PYTHON_PIP_VERSION" \
+# then we use "pip list" to ensure we don't have more than one pip version installed
+# https://github.com/docker-library/python/pull/100
 	&& [ "$(pip list |tac|tac| awk -F '[ ()]+' '$1 == "pip" { print $2; exit }')" = "$PYTHON_PIP_VERSION" ] \
 	\
 	&& find /usr/local -depth \

Dockerfile-debian.template

@@ -50,7 +50,12 @@ RUN set -ex \
 		&& python3 /tmp/get-pip.py "pip==$PYTHON_PIP_VERSION" \
 		&& rm /tmp/get-pip.py \
 	; fi \
-	&& pip3 install --no-cache-dir --upgrade "pip==$PYTHON_PIP_VERSION" \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	&& pip3 install --no-cache-dir --upgrade --force-reinstall "pip==$PYTHON_PIP_VERSION" \
+# then we use "pip list" to ensure we don't have more than one pip version installed
+# https://github.com/docker-library/python/pull/100
 	&& [ "$(pip list |tac|tac| awk -F '[ ()]+' '$1 == "pip" { print $2; exit }')" = "$PYTHON_PIP_VERSION" ] \
 	\
 	&& find /usr/local -depth \

Dockerfile-slim.template

@@ -63,7 +63,12 @@ RUN set -ex \
 		&& python3 /tmp/get-pip.py "pip==$PYTHON_PIP_VERSION" \
 		&& rm /tmp/get-pip.py \
 	; fi \
-	&& pip3 install --no-cache-dir --upgrade "pip==$PYTHON_PIP_VERSION" \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	&& pip3 install --no-cache-dir --upgrade --force-reinstall "pip==$PYTHON_PIP_VERSION" \
+# then we use "pip list" to ensure we don't have more than one pip version installed
+# https://github.com/docker-library/python/pull/100
 	&& [ "$(pip list |tac|tac| awk -F '[ ()]+' '$1 == "pip" { print $2; exit }')" = "$PYTHON_PIP_VERSION" ] \
 	\
 	&& find /usr/local -depth \

Dockerfile-windowsservercore.template

@@ -36,7 +36,10 @@ RUN $url = ('https://www.python.org/ftp/python/{0}/python-{1}-amd64.exe' -f $env
 	\
 	$pipInstall = ('pip=={0}' -f $env:PYTHON_PIP_VERSION); \
 	Write-Host ('Installing {0} ...' -f $pipInstall); \
-	pip install --no-cache-dir --upgrade $pipInstall; \
+# we use "--force-reinstall" for the case where the version of pip we're trying to install is the same as the version bundled with Python
+# ("Requirement already up-to-date: pip==8.1.2 in /usr/local/lib/python3.6/site-packages")
+# https://github.com/docker-library/python/pull/143#issuecomment-241032683
+	pip install --no-cache-dir --upgrade --force-reinstall $pipInstall; \
 	\
 	Write-Host 'Verifying pip install ...'; \
 	pip --version; \