Tourex - 11:22 am on Nov 18, 2011 (gmt 0)
You are spot on, of course. I was confusing primary keys with unique keys, hence the reason my early tests weren't giving the results I wanted. Problem solved! I can use the auto-incremented key as my primary, use a unique index on the email field as a precaution and then do a manual query before creating new accounts just as a pre-check.
Thanks for you help everyone.