input: Write a python function to find and return repeated characters in a string.
  assert find_repeat_characters('AAAAB') == {'A'}
  assert find_repeat_characters('hello') == {'l'}
  assert find_repeat_characters('testcase') == {'t', 'e'}
  assert find_repeat_characters('abcdefg') == set()
output: def find_repeat_characters(s):
      from collections import Counter
      count = Counter(s)
      return {char for char, freq in count.items() if freq > 1}