Have a look at this part of the help center: [I accidentally created two accounts; how do I merge them?](https://mathoverflow.net/help/merging-accounts)
It explains what you should to do merge two accounts.

See also: 
 * [How can one link / merge / combine / associate two accounts / users? (Anonymous / unregistered / cookie, or Google / Facebook / registered)](https://meta.stackexchange.com/q/18232) (Meta Stack Exchange)
 * [How do I combine my two accounts?](https://meta.mathoverflow.net/q/432) (Meta MathOverflow)
 * [How can I unify/merge two of my accounts?](https://math.meta.stackexchange.com/q/18786) ([Mathematics Meta]