Mastodon has a built-in system to prevent you seeing the same post boosted again and again. However, this system allows a boost to reappear after 40 other posts have scrolled through your timeline.
This worked fine in the old days when Mastodon was much quieter, but nowadays Mastodon is much busier and 40 posts can scroll quite quickly.
Here's a guide on workarounds for dealing with repeated boosts:
➡️ https://fedi.tips/how-do-i-stop-seeing-the-same-boosted-post-again-and-again-in-mastodon