Overlapping communities happen both here and on Reddit, and they happen in substantially similar ways for substantially similar reasons. Reddit has both /r/tech and /r/technology which appear to have entirely overlapping topics. There are tons of other examples of this on Reddit as well.
There's a natural incentive to avoid fragmentation, which is that empty communities aren't very interesting. Barring major disagreements about how a community is moderated, most folks will gravitate to the largest community for a given topic across all the major/commonly-federated instances.
All of which is to say, it's not obvious that fragmentation is lots more serious on Lemmy than on Reddit or other sites where anyone can create a community.