The same (or at least similar) problem occured with an earlier (3 or 4 years back) version of one of Norton's AV programs and Outlook Express. In the case of the Norton AV it happened only with incoming mail.
The AV "trapped" the incoming message, scanned it, then forwarded it. The AV apparently "made a copy" for scanning purposes, and then forward it back to the In Box, so the copy delayed by scanning then duplicated the original that went directly to the inbox. Since both Outlook and OE store messages as database records, a message is identified and stored only by record number, so identical messages (copies) don't overwrite each other, and duplicates can exist.
With the Norton, there apparently was also an ISP/Server effect, as an MSN POP3 account we had did not produce duplicates but a hotmail account did. MSN/Hotmail objected to our use of OE on both accounts, and repeatedly "broke" the account with flip-flops to "new servers" (about every two or three months for 10 years for the hotmail with about 1/2 of the "breaks" requiring both accounts to be reset), insisting that only html mail was permitted for hotmail. They eventually forced both accounts to SMTP servers rather than POP3, but I don't know if that fixed it as we'd updated the AV by the time that happened.
About 6 months ago they sent us notice that NO HOTMAIL accounts would be permitted using OE, but that was shortly after we had already switched our ISP from MSN dial-up to AT&T DSL. I still get some SPAM sent to the hotmail addy, using OE, but it's picked up by AT&T and forwarded from an AT&T server, which has been pretty much trouble-free.
My answer would be that yes, the delay caused by scanning with an AV can cause duplicate messages. In the case of the old Norton, we had no success in finding an "admission of guilt" from either Norton or the ISP, and no corrective action except via an update to the AV; but we did see it happen - but only when the ISP server also contributed to the problem.