duplicating e-mails in exchange server

hi all

We have e-mails duplicating in our inbox.
our mail server is exchange 2003.
our spam filter is GFI mail essentials
with GFI content security
Symantec antivirus 10.0

some of our e-mails are duplicating in our inbox. these e-mails are not necessarily spam e-mails. they are customer e-mails that keep repeating every 10 to 20 mins. apart from the customer e-mails we receive loads of junk e-mails that are repeated. we used to receive W32.Netsky worm in our e-mails which caused the duplication. but we have managed to remove it and block it through our content security.we have been constantly scanning our server. the anti virus does not detect anything.
can somebody tell me why this is happening??

thank you