I recently learned (after what you're describing happened to me and a quiz "randomly" (not quite!) disappeared after I copied another) that the second time you copy a Canvas quiz into the same course, it overwrites the first copy (name/instructions/questions/settings). That's how it is designed, surprisingly! This means that you can only duplicate a Canvas quiz once, and the second time you copy the same quiz it will overwrite the first copy (even if you had renamed/edited the first copy).