title: Dealing with Email address already in use - Firebase Authentication
body: <p>My app has gmail and facebook authentication integrated through Firebase. I noticed if someone signs up with their gmail then signs up with Facebook, if the Facebook had the same email as their gmail then they'll get the error:</p>
  
  <blockquote>
    <p>"The email address is already in use by another account."</p>
  </blockquote>
  
  <p>Is the only reasonable way to handle this to tell the user to sign in with different credentials? Maybe show a message like "Email already in use, please sign up with different account"?</p>