The system checks that there are no devices with the same name, I have thought this could be useful to avoid hardware duplication. Perhaps this constraint could be dropped.
Anyway, I think you should add the device inside the "printer and multifunction" category and indicate inside the description entry that that device include also a working scanner. This should be more useful since I don't think someone would buy a multifunction device in order to have a scanner, perhaps he would buy a multifunction device in order to have an all-in-one device :)
In order to add the device inside the printer category just add a space (or some other character) somewhere in the model name string, it should work ;). Anyway, I think I will drop the check upon the names.
If you add the device inside the printer category (specifying that it is also a scanner) then we could decide to delete that device from the scanner category. I think it could be confusing to find the same device under different categories.. see also this