how to change the letter that repeats itself the least to the letter that repeats itself the most code example

Example: check for double character in a string python

count = {}
for s in check_string:
  if s in count:
    count[s] += 1
  else:
    count[s] = 1

for key in count:
  if count[key] > 1:
    print key, count[key]