Disallow google/other email plus and dot tricks when registering?

You could use the user_register hook to normalize the email address and either store it in a separate field in the database or just do a query to see if the normalized email address matches. I don’t see one that is a pre-user_register hook though, so it appears that you would have to delete the user after it was created if the user is already registered.

Here is the link to the docs: http://codex.wordpress.org/Plugin_API/Action_Reference/user_register

And here is a stackoverflow question about normalizing gmail addresses: https://stackoverflow.com/questions/15592956/gmail-username-collapsing

And some code to do it:

https://stackoverflow.com/questions/1586389/how-to-check-for-a-duplicate-email-address-in-php-considering-gmail-user-name

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