Can't seem to remove a custom MAPI property (5.2.0)

In the application I am writing/using to process messages, I stamp a MAPI property to the message during output as sort of like a "watermark".  This seems to work just fine.  However, if I need to reprocess any of those messages, it appears that I can't "delete" the original watermark, and if I try to create another one, now I have two MAPI properties with the exact same name but different values.

Attaching sample message and code.