Appointments received in Mac Mail are not created properly in Calendar

I recently upgraded my Mac from Snow Leopard to Mountain Lion. Since that time, I have had a problem where appointments that I receive as attachments (Mail Attachment.ics) to email messages are showing up incorrectly in Calendar when I click on them. Specifically, the Calendar will highlight an already existing appointment that has nothing to do with the new appointment. Perhaps it has something to do with the system reusing unique IDs of some sore in the mail attachment.

Does anyone have a recommendation of what I can do to fix this problem?

Thanks,
Jay