Last night, I used a program called Mp3Diags to examine the metadata in my collection of "Oldies" music tracks. I should add that I created and tagged these mp3 files about 5 years ago. I'm fairly certain I did the (ID3v2) tagging with Mp3tag. For each track, I entered Title, Artist, Year, Album, Comment, and a couple of other fields. I also used Mp3gain on each track, which adds an APEv2 tag containing the "undo" information.
Since then, I have used Winamp, MediaMonkey, XMPlay, foobar2000, and now MusicBee to play those tracks. The metadata continues to work fine with these players to the present day. While doing some recent updating of tags in Mp3tag, I noticed that, for a lot of the tracks, the Year values were duplicated, as already mentioned. Example: Year = 1978\\1978.
Using Mp3Diags, I can now see that somewhere along the line, an ID3v1 tag has been added to a lot of the tracks, and these are the ones showing duplicated year values in Mp3tag. So, Mp3tag is showing me the Year value from the ID3v1 tag, as well as that from the ID3v2 tag.
I further discovered that, in those tracks to which ID3v1 tags had been added, the ID3v2 tags had been modified to include ReplayGain data, using frames RGAD and TXXX.
I did not intentionally or knowingly add the ID3v1 tags or the ReplayGain data to any of these tracks. I must have done it accidentally. Perhaps it was by way of MediaMonkey. These files have all been "Read Only" for several years, so perhaps I added these extra tags and frames accidentally while updating Comments, or something like that.
Anyway, I will probably use Mp3tag to remove the ID3v1 tags, as well as the ReplayGain values (I have no need of those, as the tracks are all processed with mp3gain). Presumably, the double Year issue will then disappear.
I thought I'd explain what I found, since folks were so kind about helping me with this.