Weird Virus?

I have a friend who sends emails to me, and lately when I reply to her I'll get an exact duplicate of my reply sent back to me. It's intermittent, not consistent, but it seems to happen when I reply to an email from her. It's as if she just forwarded my reply back to me, but without the "original message" tag line or the FW in the subject line. Or as if she started to reply but hit Send accidentally. The thing is, she doesn't remember sending them and there isn't anything in her Sent folder, which sounds suspiciously like a virus of some sort. Her full headers seem to show the emails as coming through the servers of the ISP she's with. I asked my ISP about this and they said it would be a virus on her computer, not mine, but the guy couldn't think of the name of the virus. I do daily full scans and it never shows anything.

I warned her about this and she did an updated scan (Norton) which didn't come up with anything. I personally don't trust Norton so I suggested she try an online scan, and the only things it came up with were a garden variety tracking cookie and TagAsaurus (which apparently gives you extra popups, not echoed emails). I can't seem to find any information about these symptoms because the only keywords I can think of usually mean other things in the same context (e.g. echo, email, reply, duplicate).

I thought if I could find out which virus (or other malware) she has, I can help her track it down and nuke it. Does this sound familiar to anyone? Thanks for any help with this.