I recently made an edit to an answer and another user raised the question of whether it was "kosher." In this case, I feel pretty confident that it's reasonable (or else I wouldn't have done it). On that question, there was a comment that contained a pertinent fact (who gave the "Moonlight Sonata" its moniker). The author of the accepted answer made reference to this comment but didn't duplicate the information. However, since someone was recently pointed to this question and said that it didn't contain the answer they were looking for, I felt it was reasonable to spell out in the accepted answer the data that the author was referencing.
I'll bring up a slightly different situation I've thought about recently (on a different SE). A user asked a question that felt like it ought to be a duplicate. A search revealed an older question that was a near duplicate, but brought in a shade of emphasis that wasn't in the new questioner's intent, or vice versa. I considered encouraging the questioner to edit the old question to allow it to serve both their purposes, but ultimately felt it would be too invasive. If I recall, it wouldn't even have violated the original intent of the old-questioner, and their questions were really the same, but the edit would have removed elements that were referenced in comments or answers, so I ultimately bit my tongue.