This issue has been reported for MDaemon v9.5.4, but may apply to other versions as well.

The Outlook connector can modify a calendar event for several reasons, one being the reminder display popup. When the Outlook connector changes an event and NotifyLink processes a change from the device for the same event, a duplicate may be created on the MDaemon server. The duplicate will then be synchronized to the device.

This is due to a combination of a bug in the MDaemon SyncML server and how the Outlook connector edits an item. MDaemon is aware of the issue and will work on a fix for a future release.