> subnet guaranteed to be unique within the scope of the template or the entire organisation?
I don't know the answer for sure.
I believe it is only unique to that specific template.
If the subnets are used in AutoVPN, and a duplicate subnet error does occur, you will get an error when clicking saving preventing the config from being used.
So depending on the degree of overlap, you might click "save" most of the time successfully and every now and then it will generate an error.