a = [7, 5, 5, 4]
b = list(itertools.combinations(a, 2))

# b = [(7, 5), (7, 5), (7, 4), (5, 5), (5, 4), (5, 4)]

index = [(0, 1), (0, 2), (0, 3), (1, 2), (1, 3), (2, 3)]