When listing multiple units on the same parcel in Matrix, you will get a yellow Duplicate Active Listing warning message when you click Submit Listing. This warning is designed to prevent agents from accidentally listing the same property multiple times. It comes up when Matrix sees that there is already at least one active listing using the same parcel ID on the system.
If you get this warning message, just click the Submit Listing button a second time to get past it and save your listing as Active. The warning will not prevent you from activating the listing.