We have a service which syncs our calender with an exhange calender. In the synchronisation process we use the unique ids to identify the appointments. Now we have a customer, which has non-unique UniqueIds.
I used the EwsEditor (https://github.com/dseph/EwsEditor) to inspect the elements and yes, both appointments (same user, same month, same UniqueId, but a different appointment) have exact the same UniqueId.
Both appointments are not created with our software. They are manually created by the user via Outlook.
Is there a reason why the exchange creates appointments/meetings with the same Ids?
Are you saying that an Id has been reused (if so that's possible as it will still be unique). Or are you saying that you have two id's in the same calendar that are the same if so are you sure your not confusing recurring appointments or that fact that the UnqiueId's are base64 encoded so this means the Id's are case sensitive.
That said using the UniqueId for calendar appointment isn't a great idea and you would be better using the GOID property like PidLidCleanGlobalObjectId https://learn.microsoft.com/en-us/office/client-developer/outlook/mapi/pidlidcleanglobalobjectid-canonical-property