iCal adds two entries for each new event

Each time someone sends me an invitation, iCal adds two entries for that one event. One is grey and the other is green (the color of my work events). When I try and delete the grey version, it asks if I want to "delete and don't notify." I say yes -- then I will get a call from the invitation submitter asking why I deleted the invitation? Why is iCal giving two entries for one event? Thanks!!