I do not see this as being uncommon, I myself have answered my own questions on probably 10-20% of questions asked because:
- Its my problem, so I will have more motivation to fix it.
- As in 1. but I have more information about the problem and the required solution, given its unlikely every bit of information is included in the initial question.
- As in 1. but as I have the problem in front of me, i'm likely to keep on researching for a solution and will have more time to do so, again because this is the problem that is preventing progress.
Personally, I see no issues with people answering their own questions as either an answer or as a comment. If I solve it, then I will write the answer and mark it as solved so if someone else has the same issue they know the solution.
However for what to do with comments stating its solved and no answer accepted, I can only suggest flagging it to a moderator to edit the question with the content of the comment or close the question with a given reason.