In addition to other reasons, I suspect people might be using the Identify page and hitting A to agree, then C to add a comment.
Perfectly fine. I do it for trips, eg https://www.inaturalist.org/projects/hastily-put-together-australia-trip
Definitely not.
No, please don’t do this. Politely ask the user to remove the observation and add the photos to the remaining observation. But “duplicates” aren’t the biggest deal, although they can be frustrating.