Let me try to understand the problem by a simple example with two files.
File 1: "File1.mp3", having one tag-field ARTIST, value "ARTIST UNKNOWN".
File 2: "File2.mp3", having one tag-field ARTIST, value "Artist Unknown".
While trying to change the filenames using converter "Tag - Filename" with Formatstring "%ARTIST%", the file system will throw a naming error, because of the duplicate file name, because the file system can store mixed case names, but does not differ between lower case and upper case.
So you are searching for some functionality to detect such files, which have the tag-field ARTIST written in any variation of lower case or upper case letters, but with basically the same text content regardless of letter case.
In general you can detect and visualize those duplicate tag-field entries simply by sorting the tag-field column. Duplicate entries will be ordered together, guided by the collation sequence.
In order to avoid file system errors when renaming files, it is best practice to assemble the new filename in a helper tag-field (I repeat myself for years here in the forum), and check this tag-field content for possible upcoming errors, before writing the new file name to the disk directory.
First assemble the new filename into a user defined tag-field e. g. "MY_FN".
Do this with all files of your file collection.
Then sort the collection by this tag-field.
Files with the same content will be obviously arranged together in the file list.
Then do what you want, e. g. change the content in the original tag-fields and such things.
If you want to see only the "duplicate" files in an ordered list output, then you can adapt the following proposal to your needs. I have just published a "Duplicate Detect Method", see there:
Wie vorgehen um doppelte per %_md5audio% finden
I admit, for the Mp3tag novice user the newly introduced "Duplicate Detect Method" looks complicated, but this is only founded in the usability of Mp3tag system variables %_crc%, %_md5%, %_md5audio%. Mp3tag calculates these values only when exporting into text file, so the values need to be re-imported for the further step of comparison.
For your task the export and import of values is not necessary.
The described process for duplicate detection can be reduced to minimum effort.
We have a bunch of files and want to examine their tag-field ARTIST for similar values, better to say, we want to check the tag-fields for the same value, which differs only by letter case.
This can be done by the following export script.
'List of files with the same value (ignoring case) in tag-field: ARTIST'
'Totals - Searched Files: '%_total_files%' - Original File Size: '%_total_size%' - Playing Time: '%_total_time%
%_datetime%' script generated by '%_app%' - <a href="http://www.mp3tag.de" target="_blank">http://www.mp3tag.de</a> -'
The script can be adapted to any other tag-field. To use it with the user defined tag-field "MY_FN" all occurrences of "%artist%" must be replaced by "%MY_FN%".