So you basically want an option for the template to prevent adding extra values for a property? Or, expanding the idea slightly, to be able to control how many there can/should be (so you could set a maximum of some higher number than one also?).
We don’t really have a way for you to add that with a module, but I think it’s a pretty good idea for a potential improvement to the template system in S itself.
I’ve created a new datatype and for this dataype, I want to modify the template to prevent adding extra values of properties (by simply hiding the “Add value” button in CSS).
I wonder where add the “unique” checkbox in the application (via a module).