list(merge_no_repeat(iter([(2, 'a'), (4, 'a'), (6, 'a')]), iter([(1, 'b'),
    (7, 'b'), (8, 'b')]), itemgetter(0)))
list(merge_no_repeat(iter([(2, 'a'), (4, 'a'), (6, 'a')]), iter([(1, 'b'),
    (3, 'b'), (4, 'b'), (5, 'b'), (7, 'b'), (8, 'b')]), itemgetter(0)))
Merge two sorted iterators without replace