ok... but when you say mirror, do you mean another hosted site mirroring the original, or something else?
Update: I noticed an option to mirror in the Manage Domains panel, I assume that's what you were referring to, so I've done that.
Further update: Yikes... just noticed this warning about mirrored sites.
Yet another update: Following the advice at the link above I've set up the original domain to disallow robots via .htaccess. Fingers crossed this avoids blacklisting.