I feel like this can, and should, be handled client-side. It would be a relatively simple matter of auto-hiding a post where the title is a duplicate match to anything in the last, say, 25 posts. User configurable.
I know that feels like a hack, but for the reasons posted by the top commenters, we don't want to make things weird for parallel communities.
Reminds me of that part in The Expanse where they irradiate everyone on the station to activate the protomolecule.
So, watch out for that.