I was told that my MP3 files have multiple tags, eg the GENRE field specifically, but I cannot see them in the main screen, nor can I find a place that shows any multiple tags for the same file. Is there a way to see this information, AND is there a way to strip all multiple tags from every file and just keep the ones I have setup that I see on the main screen?
THANKS!
you see multi-value fields at least in the extended tags dialogue (Alt-T).
If you want to filter to see if there are multiple values for a particular field, try
"$if($neql($meta(genre,1),%DUMMY%),1,0)" IS 1
Then there is an action to either merge those fields or to remove all except the first one.
See the help on actions:
When I do ALT-T - I do not see any multiple tags for any field. I also have no clue what to do with that "$if(..." code. I've attached the extended tags view of a file I was told definitely has multiple GENRE tags. Thank you for your help! 
AFAIC see it does not have multiple values for GENRE.
So the question is: who told you that?
If it is the player - well, some interpret the slash as separator. But this is no genuine multi-value field. (Other player take e.g. "semicolon blank" as separator.) But it still remains data in a single field - to get a multi-value field, you need at least 2 entries with the same field name.
The code I offered is a filter expression, you enter it in the filter box.
There does seem to be some duplication though. For example I see an Artist tag as well as Artists. And there are a bunch of MusicBrainz tags, can’t see the whole list.
But this is no multi-value field and their contents would not appear in the same column or whatever. Multi-value fields have to have literally the same name in the extended tags dialogue.
Try it yourself and click the "New" button and create a field that is already there.
Understood, those two are not identical. But if the concern is regarding duplication within some players, it does question how those similar fields are handled by the player. Same reason I mentioned the Music Brains tags.
This is what I was told...
MP3TAG does not show the "raw" embedded tags of a file, it has a UI which only knows and writes its own tags. If you download exiftool and ffmpeg, which are command line tools, just type "exiftool " and for ffmpeg it is, "ffmpeg -i " each will read and output information from the file. I hope that helps.
And here it is, using that FFMPEG tool... 
This is not really correct.
See the help for a mapping of MP3tag internal names and their fields. atoms, chunks etc:
And even in the ffmeg dump I do not see a duplication of genre
Obviously, however, there is something that MP3TAG is not showing or deleting when a tag is updated. If you are saying that MP3TAG does show the raw embedded tags, then I should be able to see TRAILER somewhere in the software for this file (and many others, unfortunately)
You could try the command
exiftool -j -b -G4 "99 Hooked on a Feeling.mp3"
and see all the values including any picture in binary format, looks similar to this:
[{
"SourceFile": "03 - Mercy and Grace.mp3",
"Unknown:ExifToolVersion": 12.10,
"Unknown:FileName": "03 - Mercy and Grace.mp3",
"Unknown:Directory": ".",
"Unknown:FileSize": "8.8 MB",
"Unknown:FileModifyDate": "2020:11:23 15:36:24+01:00",
"Unknown:FileAccessDate": "2020:11:23 15:36:30+01:00",
"Unknown:FileCreateDate": "2020:11:22 15:01:41+01:00",
"Unknown:FilePermissions": "rw-rw-rw-",
"Unknown:FileType": "MP3",
"Unknown:FileTypeExtension": "mp3",
"Unknown:MIMEType": "audio/mpeg",
"Unknown:MPEGAudioVersion": 1,
"Unknown:AudioLayer": 3,
"Unknown:AudioBitrate": "320 kbps",
"Unknown:SampleRate": 44100,
"Unknown:ChannelMode": "Stereo",
"Unknown:MSStereo": "Off",
"Unknown:IntensityStereo": "Off",
"Unknown:CopyrightFlag": false,
"Unknown:OriginalMedia": false,
"Unknown:Emphasis": "None",
"Unknown:ID3Size": 42315,
"Unknown:Album": "Collisions and Castaways",
"Unknown:Artist": "36 Crazyfists",
"Unknown:Band": "36 Crazyfists",
"Unknown:Comment": "Tagged",
"Unknown:Genre": "Metal",
"Unknown:Title": "Mercy and Grace",
"Unknown:Track": "03/11",
"Unknown:Lyrics": "Untangle me from my detest\r\nUntangle my wreck\r\nSlowly but surely were dining in\r\n....\r\n",
"Unknown:Year": 2010,
"Unknown:Warning": "[minor] Frame 'TSOP' is not valid for this ID3 version",
"Unknown:PerformerSortOrder": "36 Crazyfists",
"Unknown:AlbumArtistSortOrder": "36 Crazyfists",
"Unknown:UserDefinedText": "(Acoustid Fingerprint) AQADtGkjUVHCQDeaM9CuZ-gJG71UQV-K2MOzBN9RPkzgG9-FZHuPf...",
"Unknown:PictureMIMEType": "image/jpeg",
"Unknown:PictureType": "Front Cover",
"Unknown:PictureDescription": "",
"Unknown:Picture": "base64:/9j/4AAQSkZJRgABAQEBLAEsAAD/2wBDAAgGBgcGBQgHBwcJCQgKDBQNDAsLDBkSEw8UHRofHh0a.../9k=",
"Unknown:DateTimeOriginal": 2010,
"Unknown:Duration": "0:03:50 (approx)",
"Copy1:Comment": "(Songs-DB_Custom1) 400x400",
"Copy1:UserDefinedText": "(Acoustid Duration) 229",
"Copy1:Title": "Mercy and Grace",
"Copy1:Artist": "36 Crazyfists",
"Copy1:Album": "Collisions and Castaways",
"Copy1:Year": 2010,
"Copy1:Track": 3,
"Copy1:Genre": "Metal",
"Copy2:UserDefinedText": "(MusicBrainz Album Artist Id) 781167d7-b9e5-48ea-9f66-67c4a6739562",
"Copy2:Comment": "Tagged",
"Copy3:UserDefinedText": "(MusicBrainz Album Id) d7c4c20a-f84b-4b13-980d-b36bd143a23a",
"Copy4:UserDefinedText": "(MusicBrainz Artist Id) 781167d7-b9e5-48ea-9f66-67c4a6739562",
"Copy5:UserDefinedText": "(MusicBrainz Release Track Id) 76dcf756-9302-3a68-9832-3453dcf12207",
"Copy6:UserDefinedText": "(MusicBrainz Album Type) Album",
"Copy7:UserDefinedText": "(ARTISTS) 36 Crazyfists",
"Copy8:UserDefinedText": "(ORIGINALYEAR) 2010"
}]
Maybe this reveals the used tag for your TRAILER "genre".
You can try different -G1 or -G2 or -G3 or -G4 or -G5 options to see different descriptions for your values.
With exiftool -j -b -G1 "99 Hooked on a Feeling.mp3"
the same content looks like:
[{
"SourceFile": "03 - Mercy and Grace.mp3",
"ExifTool:ExifToolVersion": 12.10,
"ExifTool:Warning": "[minor] Frame 'TSOP' is not valid for this ID3 version",
"System:FileName": "03 - Mercy and Grace.mp3",
"System:Directory": ".",
"System:FileSize": "8.8 MB",
"System:FileModifyDate": "2020:11:23 15:36:24+01:00",
"System:FileAccessDate": "2020:11:23 15:49:44+01:00",
"System:FileCreateDate": "2020:11:22 15:01:41+01:00",
"System:FilePermissions": "rw-rw-rw-",
"File:FileType": "MP3",
"File:FileTypeExtension": "mp3",
"File:MIMEType": "audio/mpeg",
"File:ID3Size": 42315,
"MPEG:MPEGAudioVersion": 1,
"MPEG:AudioLayer": 3,
"MPEG:AudioBitrate": "320 kbps",
"MPEG:SampleRate": 44100,
"MPEG:ChannelMode": "Stereo",
"MPEG:MSStereo": "Off",
"MPEG:IntensityStereo": "Off",
"MPEG:CopyrightFlag": false,
"MPEG:OriginalMedia": false,
"MPEG:Emphasis": "None",
"ID3v2_3:Album": "Collisions and Castaways",
"ID3v2_3:Artist": "36 Crazyfists",
"ID3v2_3:Band": "36 Crazyfists",
"ID3v2_3:Comment": "Tagged",
"ID3v2_3:Genre": "Metal",
"ID3v2_3:Title": "Mercy and Grace",
"ID3v2_3:Track": "03/11",
"ID3v2_3:Lyrics": "Untangle me from my detest\r\nUntangle my wreck\r\nSlowly but surely were dining in\r\n...\r\n",
"ID3v2_3:Year": 2010,
"ID3v2_3:AlbumArtistSortOrder": "36 Crazyfists",
"ID3v2_3:UserDefinedText": "(Acoustid Fingerprint) AQADtGkjUVHCQDeaM9CuZ-gJG71UQV-K2MOzBN9RPkzgG9-FZHuPfFo...",
"ID3v2_3:PictureMIMEType": "image/jpeg",
"ID3v2_3:PictureType": "Front Cover",
"ID3v2_3:PictureDescription": "",
"ID3v2_3:Picture": "base64:/9j/4AAQSkZJRgABAQEBLAEsAAD/2wBDAAgGBgcGBQgHBwcJCQgKDBQNDAsLDBkSEw8UHRofHh0aHBw.../9k=",
"Unknown:PerformerSortOrder": "36 Crazyfists",
"ID3v1:Title": "Mercy and Grace",
"ID3v1:Artist": "36 Crazyfists",
"ID3v1:Album": "Collisions and Castaways",
"ID3v1:Year": 2010,
"ID3v1:Comment": "Tagged",
"ID3v1:Track": 3,
"ID3v1:Genre": "Metal",
"Composite:DateTimeOriginal": 2010,
"Composite:Duration": "0:03:50 (approx)"
}]I used exiftool and it only showed the one genre tag that I placed on it in MP3TAG. Only ffmpeg is showing the TRAILER tag. And the music service I am using used that tag instead of the one I wanted.
Also, I have 18000 mp3 files. Doing this one by one isn't feasible....
Could you please post the complete -G4 output for your file?
This just helps to identify the value. It's not intended to use it for 18'000 files. 
you asked for it...
1.zip (304.6 KB)
That's strange, no hint about "Trailer" 
Could it be something local for that music service and that it is actually not stored in the tag?
I did not find any hint for "Trailer". And neither MP3tag nor Foobar2000 show anything like it.
It's because ffmpeg reads the 70 from "70s Rock/Pop" as genre number and not as text. Genres can also be stored as numbers and 70 stands for Trailer.
Now THATS some good info! Any chance it read's "80" as the word FOLK, because that's another screwup in my upload.
FWIW I am using iBroadcast now that google play music is deceased.