If 2.4 server is started with not properly downgraded 2.6. DB files the user document format it not recognized which is expected behavior.

However if there are multiple users with the same name in different DBs the 2.4 won't start at all complaining that the unique index invariant has been broken on the admin.system.users collection.

Make this error message more clear indicating a version mismatch between the server and the DB files. Given how long 2.4.x will be around it might make sense to get this in place to avoid confusion.