There is no harm
Two (or what your suggestion implies, N) nearly identical interesting things on the front page is less interesting than two different interesting things on the front page.
This is, in fact, a problem that afflicted HN more or less right off the bat as it does any crowdsourced link aggregator - Erlang days, etc. It is, as you say, entirely predictable and crushes interestingness.
Though unforeseen consequences of this suggested policy are possible I think they're unlikely and I don't believe anyone can reliably predict them without trying it.
As dang likes to point out, internet discourse is mostly stateless. Combining duplicates that aren't really duplicates does nothing to improve this situation.