title: Firebase user merge auth providers at login
body: <p>In my Angular 2 app, I want to allow the user to connect through email/password, Google and Facebook providers. </p>
  
  <p>At first login, i understand a user entry is created in the firebase database. For example <code>facebook:UUID</code>. If the user login using another provider, I understand firebase will block this action, especially if the email is already used and has been set to unique in rules.</p>
  
  <p>So my question is: <strong>how can we AUTOMATICALLY, being transparent for user, merge the data if we detect the email is the same for example</strong>.</p>
  
  <p>On such great websites as udemy, or other big companies, we can auth using several methods. I tried to login using both Facebook and Google (both using the same email) and they redirect me automatically to my unique profile without any action required from my side to confirm i want to link my account with other provider.</p>
  
  <p>How can this be achieve in Firebase? <strong>Do we have to replicate data</strong> and make our own "user data gathering"?</p>