If a user selects pay later or starts a payment processor transaction and doesn't complete it their registration becomes 'pending'. They should really be able to return to this pending enrolment and complete (once they've gotten hold of mummy's credit card or whatever) but they just get :
Oops. It looks like you are already registered for this event. If you want to change your registration, or you feel that you've gotten this message in error, please contact the site administrator."
And can't progress any further