Error: Can't Set Headers After They Are Sent

I'm trying to do a login function but the Error: Can't set headers after they are sent is keep showing up in my terminal, based on my understanding this error is causing by res but

Solution 1:

It looks like the callback passed to validateLogin is called twice:

• once from validateUserDoc;
• once from the process.nextTick callback (that part seems to be some left-over code);