Maybe I’m missing something, but if two observations have different photos, I generally don’t think of them as “duplicates.”
Although if they are different photos of the same individual organism taken at the same time, then yes, I agree that they should have been posted in a single observation, and that merging them with comments and IDs might result in unintended association of the comments and IDs with the newly merged set of photos.
So might be best to implement at the same time as this feature request:
https://forum.inaturalist.org/t/have-system-add-a-comment-when-photos-or-audio-change/14985