When you cross-post something this happens. But in the background the Lemmy web-ui is just comparing the URL and if it is exactly the same it treats them as a cross-post.
There is quite a chance for this to backfire isn't there?
For example if user 1 posts an article with the title "This is bad oh no what do you think" and user 2 with "This is what happened [...]", there shouldn't really be a merge just cause it's an identical link imo