It appears that somehow the composer tag has been flattened (no longer a multi-value tag) prior to the $copymerge() function being applied. Before performing the $copymerge() try putting $setmulti(composer,%composer%) and see if that helps. You may also need to add $setmulti(lyricist,%lyricist%) and $setmulti(writer,%writer%) before $copymerge() as well if those tags have also been flattened.
The $copymerge() function will automatically remove any duplicates.