There are many communities that are similar on different instances, firefox for example.

Sometimes they are identical and should merge, sometimes due to the specific audience of the instance, it’s better to keep them separate.

I wouldn’t suggest merging !movies@lemmy.world and !movies@hexbear.net

Would be nice if lemmy supports something like multi Reddit with the option of hiding duplicate posts.

Mbin does support multireddits, but this doesn’t seem to be interesting enough for people to switch to it (while Lemmy communities are fully accessible from Mbin)