So I've done some testing with duplcate tags with different values...
Reading a tag only acts on the first tag (also interesting that the Column display uses a different duplicate frame than Action uses), so I can't read them to a temp location. Writing to the tag also clears all of the other duplicates, as I tried to see if I could read and clear one at a time.
I would appreciate it if we had a Merge Duplicate Fields action. Separate them with a semicolon or forward slash (commas are too frequently part of the original values). I can easily S&R this with a second action.
In the long term, you might consider full support for multiple instances of the same frame. I noticed that the ID3v2.4 implementation has several frames that are intended to be used as many times as necessary.
Thanks for a great product.