I can see this may be of privacy concern for some site operators, but XenForo by default is already announcing the email address is already in use. This is just a request to add a link to recover password to that announcement that already exists.
This is the current automatic response when attempting to register with an email that already exists for a user:
[ATTACH=full]257257[/ATTACH]
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Now if you're worried about privacy, you could make a separate suggestion (if one doesn't exist already) that the above response be changed or removed altogether.
Then the solution may be to automatically send an email to the existing address notifying the user "Someone has attempted to created another account using your email address. If you need to recover or change you password go here...."
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As XenForo currently exists I think adding a recover password link to the existing duplicate email response would significantly reduce users creating secondary accounts.
