Hi,
I manage a network running SBS2003 with 30 XP Pro clients running Office 2007. Back in December 2010 user1 sent out a meeting request to User2. User2 accepted the request & User1 received the response.
Since then User2 has been automatically sending out the same reply to User1, several times a day. By automatically I mean that the user is not sending the reply, but Outlook is automatically sending out the reply. I have seen it send a reply whilst connected to the client by RDP & there are dozens of reply's in the "sent items" folder of User2.
There is no pattern to the sent replies, sometimes minutes in between & sometimes hours. The replies stop being sent when Outlook is turned off.
I have tried the following,

I have accepted the original request again.
I have deleted the original request & the calendar entry that it created.
I have scanned the .OST file with the scanost.exe tool.
What could be causing this?