If you receive calendar invitations by e-mail and have Outlook set to automatically add them to your calendar, Outlook may see a good message that SpamSieve moves back from the InboxSpamSieve folder to Inbox as a new invitation. To prevent a duplicate calendar event from being created, you can set up your Outlook rules to not move the invitation e-mails to InboxSpamSieve. For example, you could add a rule above the SpamSieve rules that moves the invitation messages to another folder.