User account creation safety and race condition concerns. Mongodb + nodejs

First question here :D

I am trying to deal with an edge case with user account creation using node.js and mongodb, where user or users attempt to create an account with the same username. When a new user attempts to create a new user account, I first check if it exists in the database using findOne which is an asynchronous call. Upon finishing finding one, there is a callback which calls another async call to a bcrypt subroutine.

I am scared of the following execution sequence:

req->req->findOne->findOne ...

or even:

req->req->findOne->bcrypt->findOne ...

How can I deal with such a situation?