Duplicate invitations to a meeting

I am desperate. Some application is repeatedly sending my meeting invitations without my intervention. I don't want to spam my colleagues & I didn't find any solution to this problem.

I access the Exchange account via: iCal, Mail, Outlook for Mac, Calendar & Mail on my iPhone. I plan meetings & invite participants always in Outlook for Mac, the iCal app is running simultaneously. Some meeting invitations are sent later again without my intervention: I do not edit the meeting in the calendar, I'm not working with calendard & somehow the invitation is send again, it occurs in my sent mail folder. Even for meetings in the past. Unfortunatelly I didn't found any pattern in it.

Thank you for any advice.