def combine_lists(lists):
    # Keep a set of processed items
    skip = set()
    sets = map(set, lists)
 
    for i, a in enumerate(sets):
        # If we already returned this set, skip it
        if i in skip:
            continue
 
        for j, b in enumerate(sets[i + 1:], i + 1):
            # Use a set to check if there are common numbers
            if a & b:
                skip.add(j)
                a |= b
 
    # yield all sets that were not added to different sets
    for i, a in enumerate(lists):
        if i not in skip:
            yield a
 
print list(combine_lists([[13, 15, 17], [1, 2, 4], [1,7,9]]))

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