I found what seems to cause this issue. Since the MP3TAG program uses the "\\" to create "virtual duplicate fields" they are not yet considered "real". For this reason the Remove Duplicate Fields doesn't work.
Well, at least I just found a workaround in the mean time:
Since "\\" will be used only to create those "virtual duplicate fields", I can run through all my Tags separately with a "Replace with regular expression" using "\\.*" (without the quotes).
Once all field modifications were done -> Click "Save". (***This is important, it will force the creation of the single tag field for all files)
Also I recommend doing this "Replace with regular expression" on each Tag Fields needed separately, the "_TAG" and "_ALL" don't seem to work for this workaround. (After the first occurance found it seems to skip all other Tag Fields afterwards... so far... but I didn't research deep on that )