I think, you have to do it manually as MP3tag does not compare files and does not rememeber what the contents in the previous file was.
And MP3tag does not look ahead to see what other data might be in a field in other files.
With a little brain work it may be possible to write an export that helps you to get that list. But that means you have to fiddle about with text files, exports, imports.
I would think that the manual way would be fastest.
And isn't this the same approach as in this thread:
Merging multiple genres into one
only this time with artist instead of genre?
I have totaly forgotten about that topic. My bad.
Thank you very much