Make sure we don't subscribe twice to same stream

LibCarla/source/carla/streaming/Client.h

@@ -33,6 +33,8 @@ namespace streaming {
       _service.Stop();
     }
 
+    /// @warning cannot subscribe twice to the same stream (even if it's a
+    /// MultiStream).
     template <typename Functor>
     void Subscribe(const Token &token, Functor &&callback) {
       _client.Subscribe(_service.service(), token, std::forward<Functor>(callback));

LibCarla/source/carla/streaming/low_level/Client.h

@@ -46,11 +46,14 @@ namespace low_level {
       }
     }
 
+    /// @warning cannot subscribe twice to the same stream (even if it's a
+    /// MultiStream).
     template <typename Functor>
     void Subscribe(
         boost::asio::io_service &io_service,
         token_type token,
         Functor &&callback) {
+      DEBUG_ASSERT_EQ(_clients.find(token.get_stream_id()), _clients.end());
       if (!token.has_address()) {
         token.set_address(_fallback_address);
       }