Considering the post (and answer) here and here and here and here, I suspect (but not sure) that, user2640560, user2626657, and user2648898 are the same person. Is there any mechanism in SO to verify that? Also, is it okay to have multiple account in SO?
No, there is no way that you (as a normal user) can verify this other than by asking the person. And they could always lie to you.
Moderators, however, have tools at their disposal to investigate this. You can solicit their attention and ask them to do so by raising a flag on one of the suspected user's posts. Select the "other" option and write some text explaining what you think is wrong. It would be helpful to provide a link to the other profile, as well.
But it is not strictly forbidden to have multiple accounts. The only thing that you cannot do is have multiple accounts that interact with one another in order to game the system (e.g., vote for each others posts, accept each others answers, etc.) Since that is what you suspect to be happening here, it would be appropriate to ask a moderator to take a look.