Problem with mail that is non-spam and also mcp

[Kenny Of The Fells]

I have a rule such that non-spam to a set of users is quarantined:

To:             pupil*@      store

This works as expected. However, if mail is also mcp, it is quarantined 
twice. Given that if I change the above rule to:

To:            pupil1*@    deliver

mcp mail *doesn't* get delivered (it does get quarantined as mcp), why is it 
stored in the first case above rather than being ignored? How do I prevent 
non-spam mcp mail being stored twice? My MailScanner.conf is set to do mcp 
checks first.

Thanks

Peter