In this case:
If it behaves like the Original Poster, then it probably is the Original Poster.
Sometimes a new user asks a question using unregistered account. After a while some substantial edit suggestion from another user appears. Nicknames are at least similar and this second user behaves like the Original Poster (answers comments, adds relevant information, narrows the scope etc.). Let's assume the second account is registered.
I can see this suggested edit in my review queue.
My point of view
I'm almost certain there is the same real person behind both accounts, still I have no evidence. All I can see is the second account "quacks like the OP".
I have no doubt I should give them some useful feedback. Last time it happened my comment under the question was like:
I believe you tried to edit the question. See I accidentally created two accounts; how do I merge them? Merge the accounts, then you will be able to edit freely because you will own the question.
I tend to give the user some time (I neither accept nor reject, nor skip the suggested edit). After a while, if the accounts are not merged (yet), I reject the edit, unless it's already accepted or rejected. I can mark "this edit deviates from the original intent of the post" as a reason.
If I'm wrong and the accounts are merged eventually, the user can edit again (as the question owner this time).
This last time such edit was accepted. It so happened I disclosed my rejection in comments and two high-rep users (+10k, +25k) argued that "accept" was the right thing to do.
I'm not sure because this is what we can read about merging accounts [emphasis mine]:
After you contact us, the Stack Exchange Team will reach out to verify that you own both accounts. If we can confirm your ownership, we will initiate a merge.
From this I understand some verification and confirmation is required to tell there's one and the same real person behind multiple accounts. Yet it seems to me some users think it's enough to "quack like the OP".
The accepted edit did make the question better, I admit. Should it matter? With this attitude we can take whole bunch of questions that are broad, lack some information etc. and make them "better" by making up and adding new details. We don't do this. Instead we do reject edits for the following reason, if it applies:
Even edits that must make drastic changes should strive to preserve the goals of the post's owner.
- What verification and confirmation is required to merge accounts? (especially unregistered with registered one).
- Do users with enough reputation see something (e-mail? IP?) that allows them to reasonably tell apart the genuine Original Poster with another account from an impostor?
- I can't see anything like it (do I miss something?). Should I perform a duck test then? Should I accept edits from users that "quack like the OP"? What is the right thing to do?
Secondary (out of curiosity; these questions may be left unanswered):
- When accounts A and B are merged, will B's pending edit to A's post be automatically accepted? or will it still occupy the review queue? (This question suggests the latter but it's old; some automation may have been added.)
- Can moderators merge accounts without explicit merge request? Do they?