How to prevent multiple accounts with social media login?
<p>Next to a regular username/email/password system I want to have social media buttons for signing in. But how do I prevent users from creating multiple accounts?</p>

<p>Let's say someone logs into my app through <code>Facebook</code> first and does things in the app that get stored in the database attached to this account. A view weeks later he want to look it up again but presses the <code>Google+</code> sign in button and now it appears he lost all his data since this account is not "linked".</p>

<ul>
<li>I could create an option for the user to link there accounts but how do I verify this? Besides that I have to merge all data in the database.</li>
<li>Perhaps the API's have build in functionality for this?</li>
<li>Is there somekind of social media library that handles this?</li>
</ul>