Timer countdown might be more complicated than necessary; I always lean toward “whichever configuration gives the user maximum choice”.

What about showing “A, B, and C reposted X” as a single post, at the time stamp of the most recent repost?

This way, you can freely repost high quality notes to boost visibility, but a “spammy” reposter can’t do anything beyond hacky-pinning something to the top of your feed, rather than filling up the feed entirely. And you can unfollow a user who is abusing the feature.