I came across a strange problem with Outlook Calendar recently where every appointment which was added to the Calendar and then edited suddenly repeated forever! A scary concept! It seemed to be related to a piece of software called 'OutlookAddinSetup'. From what I've read (http://www.eggheadcafe.com/software/aspnet/29012397/re-recurring-calenda...), this seems to be something which comes installed with Dell computers. After I uninstalled this program (from Control Panel, Programs, Uninstall a program) the problem has stopped and events only occur once. If anyone knows what the program is supposed to do please drop a comment below :)