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 collections.ordereddict in python 2.7:
from collections import ordereddict
foo = mppmt
print .join(ordereddict.fromkeys(foo))
printing
mpt