How to *not* forward "Already on the subscriber list" to the list owner?

Can Smartlist be configured to *not* forward messages with the "Already on the subscriber list" x-diagnostic to the list owner? As a list owner I don't think I need to see this message every time someone tries to subscribe twice. Don't ask me why, but with my list this happens quite often. I think this event can safely be ignored. Regards, René Pijlman