This problem comes up often: An Outlook user has duplicates of a meeting which the attendees have accepted and is looking for a way to delete the duplicates without sending cancellations notices.
The solution is simple, using an often forgotten feature: Work Offline.
Take Outlook offline (and disable the network connection as insurance) then cancel the extra meetings. Go to the Outbox and delete the cancellation notices before taking Outlook back online and reconnecting to the network.
If you don't use Send Immediately and don't use an Exchange server account, working offline may not be necessary, but going offline is a guarantee that the cancellation notice won't be sent accidentally.