Multiple emails sending from Outlook XP

I have two users who have experienced multiple messaging: they sent one message to a list of email recipients and ended up sending 6-12 messages to each of them. One user’s unique email quality was inserting a graphic. The other simple included textand an attached word doc. Scanned the first user’s PC for virus and found none. Been to Microsoft’s call-in and they’re ???

Both users experienced this problem by sending to multiple addresses. There is only one message in sent folder.

The second user even received a couple of phone calls about multiples WHILE the message was in still moving from her outbox.

Office XP running on Windows 2000, SP2 and a Dell Inspiron 4100.

Clues anyone; asker motivated…