The easies way to assign the same data to several files is to use the tag panel.
I don't see any other way in MP3tag.
So, if you have a (play)list of alleged duplicates, you would still have to select the grouped files manually (or with a filter) and then assign the data.
The next problem: what qualifies a duplicate?
Here is a lengthy discussion
and perhaps it is quite allright if the data in the tag fields stays as it is.
E.g. I know a number of files that have the same data in TITLE but are actually different as one is the normal pop version and the other the Salsa version... So, unifying the genre would then be a mistake actually.