Prevent guest orders from registered customers

Happens every now and then:

A customer registers, buys several times, becomes a regular customer and maybe even has discounts.

One day he is in a hurry and simply buys as a guest.
What happens? He gets the guest prices (and grumbles, of course).

This module checks the email address and gives an error message,
if someone tries to order as a guest although he already has a customer account.

The same error message appears if someone tries to register as a customer although he is already registered with the same e-mail address.