这是一个itertools.groupby解决方案:

from itertools import groupby
from operator import itemgetter

li = [('x', 'y'), ('s', 'e'), ('s', 'a'), ('x', 'z')]

[p for k, g in groupby(sorted(li, reverse=True), itemgetter(0)) for p in reversed(list(g))]
# [('x', 'y'), ('x', 'z'), ('s', 'a'), ('s', 'e')]