Find “one letter that appears twice” in a string

You need use capturing group based regex and define your regex as raw string.

>>> re.search(r'([a-z])\1', 'ugknbfddgicrmopn').group()
'dd'
>>> [i+i for i in re.findall(r'([a-z])\1', 'abbbbcppq')]
['bb', 'bb', 'pp']

or

>>> [i[0] for i in re.findall(r'(([a-z])\2)', 'abbbbcppq')]
['bb', 'bb', 'pp']

Note that , re.findall here should return the list of tuples with the characters which are matched by the first group as first element and the second group as second element. For our case chars within first group would be enough so I mentioned i[0].