I believe we have this narrowed done to an ACT! 2009 and Outlook issue. We have 30 users that have individual copies of ACT. They use Outlook 2003/2007, and Outlook Anywhere to retrieve emails. The backend is Exchange 2007, with latest patches.
This is the forth time this specific issue has hit us. A user will accept a meeting, and teh meeting will start to book itself over, and over again. The last user it happened to, had the meeting booked 4800+ times. In turn this locks up Outlook, and crashes the Exchange server. So far the fix has been to stop copying the Outlook calendar to ACT, adn removing the meetings from both calendars.