People always complain about this but Lemmy does actually combine crossposts in your feed. What frontend are you using? Or maybe it's just bugged? If someone can figure out the bug and how to reproduce it, it needs to be submitted to GitHub.
Here's a reference to the feature, a fixed bug, disabling the deduplication for a single community view https://github.com/LemmyNet/lemmy-ui/issues/2104#issuecomment-2316048658