Looks like a bug in the WP encrypt plugin you’re using. That error message comes from the Boulder function NewRegistration, which function is explained as: “// NewRegistration is used by clients to submit a new registration/account”.
In the block of the error message, the error message comes with a Location header, as far as I can see pointing to the registration key in question.
So if your LE plugin already has the registration key, it just should use it in stead of trying to register it again.