This is the 4th most voted feature request: Duplicate removal tool/report (including merge)
In there, there’s also a reference to a script (GitHub - elias123tre/bitwarden_find_duplicates: Find duplicate logins based on domain, from Bitwarden export. Open source for your safety.) that can help you do the manual cleanup, as it looks like the BitWarden devs are too busy to implement anything simple like that.
Sorry for necro-posting, but this is one of the (many) results about this topic on Google, which I tend to open several entries as some of them might have a better idea than the other.