Consider this POST. I think the OP has two accounts one and two.
The user edited their own POST with second account, so that it comes under Suggestion edit
Even though I know the edit is from the same user I rejected the edit and made a comment on their POST.
My question is Is my behavior is valid? What should be done to the user? Can I report to this diamond moderator, that the user has two accounts?
For users who are not diamond moderators is difficult to understand if the two accounts have been created from the same person, or different people. It could also be the accounts are created from two co-workers working on the same project.
What you have done is generally the better approach. Accepting the suggested edit could just cause the user to ignore the problem with the accounts, since they would be able to edit their questions (even indirectly). That would not resolve other problems, such as the inability of accepting answers, comments written as answers, etc. Leaving a comment is fine; I would also point them to I accidentally created two accounts; how do I merge them?
