You may have seen this: a new user asks a story-identification question, then a second (also new) account gives the answer (or confirms an existing answer, though usually this deleted and transferred to a comment). There is reason to believe that these two accounts are the same person for one or more of the following reasons:
The second account acts like they asked the question, such as by saying “Thanks for helping me find the answer to my question”.
There are varying degrees of this. Sometimes the user even explicitly admits they lost the first account
They have similar/identical usernames
They have the same profile picture
When this is an automatically generated Gravatar, this means the same email was used for both accounts, though a mismatch between Gravatars doesn’t necessarily mean the emails are different
Moderator-only information matches between the accounts (IP, email, or something else I don’t know about?)
Most of these answers are also upvoted by the community as a good match.
I already ask the user to merge their accounts when I have any reason to suspect they’ve created a second account but it’s rare that any of them stick around or respond. Beyond this, there’s not anything that even a moderator can or should do.
What reasons, if any, are strong enough for us to allow questions like this to be closed (or closed against) as a duplicate? The current policy (Closing Story-Ident questions as duplicates (where there's no acceptance)) doesn’t address this.
It’s pretty rare that this happens and a suitable duplicate also exists, but it does happen. For example, a question was recently asked and has an accepted answer but I’m not sure if this is a suitably accepted duplicate: Book series my dad told me about. It involves space travel, clones, and an indigenous people with kangaroo tails