There are several possible reasons that you may be getting duplicates of SPAM.
- You have an email alias. If you have an email alias, it is possible that spammers have harvested both your four- or six-character username and your email alias. If this is the case you should be able to tell by looking at the to: field in the email header.
- Graylisting. Sometimes spammers send duplicate messages to get around an mail server spam control technique called graylisting. In graylisting, a mail server rejects mail that comes from an unknown server, as it could potentially be SPAM. To combat graylisting, spammers send two emails. The first message is rejected by the receiving mail server because the mail originates from an unknown server; the second message sent by spammers will be accepted, because the originating server is no longer unknown. Lehigh does not use graylisting, which could be why you are getting duplicates.
For more information see the graylisting article in Wikipedia.