[[('a', 'b'), 1], [('b', 'c'), 2], [('a', 'b'), 5]]

s = [[('a', 'b'), 1], [('b', 'c'), 2], [('a', 'b'), 5]]

import collections

c = collections.defaultdict(list)

for t,i in s:
    c[t].append(i)

result = [(t,sum(v)//len(v)) for t,v in c.items()]

print(result)

[(('a', 'b'), 3), (('b', 'c'), 2)]