Ubercart's default behavior when the registration process encounters an already existing email address is to apply the order to the account that uses that address. This makes a certain amount of sense, but not with the system I'm building.

I'm selling online memberships, so the whole point is to create unique accounts. I would like to know how I can alter Ubercart's behaviour to force it to abandon the transaction and throw a clear message why, when it encounters a duplicate email address.

Any thoughts?