User registration - multiple social accounts

Andrew,
What you describe can definitely be a problem, and there are a few suggestions we can provide, but no fool-proof methods to prevent a user from getting 2 accounts.

1) If you have Automatic Registration enabled, consider turning it off. Then, if you select "JFBConnect" as the Registration Component, the user will be brought to a page where they can login to an existing account or register a new one. For returning users, they just have to login to the account they already have. The Registration side might sound like a turn off, but you can actually disable every field from showing (username, name, email and password) so that all that information is generated by JFBConnect automatically. Then, you basically just have a page that says "Login or click this button to create a new account" and there are no fields to fill out.. it's just a quick pitstop on the registration flow. There are language strings in JFBConnect you can use to change the message however you want.

2) The SCLogin module has the option to show the Connect buttons to a user that is already logged in. That will show any social networks the user hasn't already associated with and can be a good graphical cue to link their accounts (you can see this in our My Account menu in the top right).

3) you can create a custom page for "Connect your accounts" and direct users to it. It can be a Joomla article with a description of the benefit of connecting your accounts, etc, etc. On that page, just add the tag:That will show any accounts the user hasn't connected with. You can even limit the providers shown if, say, you really want to get more users to connect their Twitter and Facebook accounts only, even if you have more networks configured.

With all that said, the 6.3 release (due out later this year) will have a new page similar to option 3 above which will let users make and break connections to social networks. We're also planning on adding similar functionality to the Profile pages of many of the extensions we support, like JomSocial and EasySocial and Joomla's standard profile page, so that users will be able to see and manage their connections easier that way.

I hope that helps give you some ideas, but feedback is definitely welcome. As we add more providers, this is definitely an area we realize we need to tackle.

Thanks,
Alex