If order does not matter, you can use
"".join(set(foo))
set() will create a set of unique letters in the string, and "".join() will join the letters back to a string in arbitrary order.
If order does matter, you can use a dict instead of a set, which since Python 3.7 preserves the insertion order of the keys. (In the CPython implementation, this is already supported in Python 3.6 as an implementation detail.)
foo = "mppmt"
result = "".join(dict.fromkeys(foo))
resulting in the string "mpt". In earlier versions of Python, you can use collections.OrderedDict, which has been available starting from Python 2.7.