Hiya,
If the user has the same email registered across multiple accounts (Facebook, Twitter, Google) they should be able to login with each of those and they'll be all tied to the same account in FusionAuth. If they are different email addresses, we currently have no way to reconcile them.
But I think the user has to go through and sign in with each provider for FusionAuth to get the metadata into its system.
These github issues might be of interest:
https://github.com/fusionauth/fusionauth-issues/issues/1
https://github.com/fusionauth/fusionauth-issues/issues/751