Yes it is allowed to stay, but sometimes posters wonder why nobody is commenting on their awesome post and don't realize that there have been numerous earlier posts, and that they might benefit from the earlier comments that they missed, so , generally, if I can , I try to let them know. Anyone is free to add new posts with new evidence but they should point that out in the title if they don't want to be ignored. The rule is not in the submission requirements, its simply that most voaters here ignore stuff they've seen a zillion times before. The deletions are probably by user.

I have seen mods allow a (re)post to stay if it's something that we haven't discussed for a while, or if they deem it important enough to remind everyone about. There are no hard rules about this, which is why it's so maddening. So much evidence came up in the first few months, no one could keep up with it, so for this reason I think we should do away with mods deleting reposts, and allow the community to downvote it if it really doesn't help.

I think it depends on the reason for the new post of an old topic. There are a LOT of posts that have been covered countless times but presented as new evidence. The catch 22 there is maybe nothing new comes of it but then again, because many people sort by 'new' they would never have seen the discussion in the first place.

Depends. Is there new evidence or a new lead?