I use MusicBrainz Picard and Mp3tag.
Well, Picard saves tags as 2.4, but I used to convert them back to 2.3 for compatibility reasons. Many of my albums have a yyyy-mm-dd format in their YEAR tag (saved by Picard).
Recently, I used Mp3tag to convert my entire library to ID3v2.4 and most of (strangely, not all) the tracks that follow the format mentioned got split in two YEAR tags: one being mmdd and the other, yyyy.
Now it's pretty messy...
And, hum, if someone could provide me a script for fixing those, I'd be really grateful. (I'm working on it, btw)
Edit: I managed to write the script