I wonder if it might be a matter of communication with the customer. Reword the error message to gently remind them they have an account and that it's more convenient for them to use it now that it's already created than to not use it.

But there really should be a way for them to say, in effect, "Yeah, I know, but I want to order without logging in for my own peculiar reasons, so get out of my way and let me do it!" In other words a Cancel button in the error message which lets them proceed.

Now if only I could PHP I'd program that and offer it up.

Rob