Recipient receives multiple copies of same email through Outlook 2003

Sometimes, not always, when sending emails through Outlook 2003 (through webmail error never ocurred so far) the recipient receives from 10-20 copies of the same email. Ive scanned all drives with several antivirus software (NOD32, Panda IS 07, Norton IS 07)and found a netsky and nimda viruses altough i removed them, same error ocurred so i decided to reinstalled all machines (9 in total) and start from scratch (although that was almost impossible because people needed to use old data), anyway the same error appeared again. I rescaned with NIS 07 and NOD32 but nothing was found. Any idea what could cause this?