How do i stop multiple emails that are the same?

[wdriver]

Hi, my clients continually get 30 > 100 of the same message. These messages get straight through Spamassassin untouched.

Does anyone know how to stop these messages getting through? Maybe there is a setting somewhere to say only let 2 though and then no more (because it realises they are multiples).

Thanks for your help in advance

Wayne.