$copymerge removes duplicates. It seems rather convoluted to come up with equivalent functionality that allows duplicate values to exist.  I think there should be a copymerge function that allows duplicate values.

Example use case - maintaining 3+ parallel arrays (multi-value fields) with the same number of items:

musicbrainz_artistid: 2eb7e63d-c1fa-4308-b918-c539f21cebea; c1970887-9e13-4f24-aa87-d8cacc3b0690; 74c3e92e-23a7-4d71-bf4c-e57e885ad77d; 624ec66b-4d6f-46c6-89d6-eeed220d540f
artists: 一条蛍; 村川梨衣; 越谷夏海; 佐倉綾音
artists_type: character; person; character; person

Currently, $copymerge would result in just "character; person" rather than the required "character; person; character; person"

The same issue would present itself if the same Artist MBID occurs more than once in an artist credit, which is exceedingly more likely with aggressive merges of name variations that might be separate projects.