Exactly duplicating another answer
This specifically came up for me in a king-of-the-hill contest. In particular, this contest had a random component, so replicating a submission could allow one to win by luck.
In general, duplicate submissions are uninteresting, and do not add anything to any sort of contest, king of the hill, code golf or otherwise.
Importantly, even if the posters came up with the answers separately, duplicate answers should still be banned.