Not just possible, but for files that use vorbis comments for metadata, such as FLAC and Ogg Vorbis, it's the recommended way to have a field with two or more values. There would be no reason to have two with the same value, and probably no reason to use it in the YEAR field. But for things like ARTIST and GENRE, it's fairly common.
ARTIST=Tony Bennett
ARTIST=Bill Evans
GENRE=Jazz
GENRE=Vocal
So, yeah, fix them. To find them you can use a filter. Load your whole library, hit F3 and enter:
"$len($meta(year))" GREATER 4
That should find anything with two or more YEAR fields (and anything with a date containing more than four characters.)