There are several reasons why you can encounter that kind of situation, so we'll look into each of them separately:
Your creative will always be duplicated if:
You're promoting a post, but you're using placements that differ from the original one (for instance you're using a Facebook post, and promoting it to Instagram, it usually duplicates when adding to the IG Feed).
If you're adding the Instagram of Facebook Stories placement, it will always duplicate your creative.
If you're using a tracking strategy that rewrites the URLs
When you are editing a creative, you are not actually changing the existing post, but you will duplicate the post, and edit the new one. That means you will end up with 2 posts, and your engagement will be split.