Partly this is why there is the message advising users to sign in - but beyond that, if the email address entered already belongs to an account, it won't let them continue.
@Mark I'm also curious about how you handled this scenario. We actually wrote our own mod that does almost exactly what you've announced in this topic (create a temp account as soon as they submit the post, and leave it un-approved until they confirm/complete registration). It worked as intended (increase registration and engagement - but we ran into issue with existing members posting while logged out, and ended up shutting off the mod until we could get back in and address a way to merge the "temp" account created with the new post to the pre-existing account the member already had (but wasn't logged into at the time they made the post). Once we started running it live, we ran into the issue of members existing accounts, and ended up turning it back off until we could address the issue. After that, life happens and we never got a chance to go back in and work out a solution for that situation.
I'm crossing my fingers that you accounted for this in your system already?