Currently we have a site that uses basic site login using a username and password that is stored on the server. Our client wants to introduce Facebook login alongside the current site login.
If a user already has a 'site' account then decides to signup with Facebook then we can match the two up if the email is the same (as would be the case in most scenarios) - The problem is introduced when the user has a different email for their Facebook account / site account. This would then leave the user with two accounts and could potentially lead to issues (users earn points for each purchase).
Has anyone had any experience with this and if so what are the pitfalls to avoid / best practice for dealing with this.