If I'm reading this right all this does is define the first extension as the primary extension, which still has to be unique. So they would still have to use a different extension as the first one in the list, it just wouldn't be called "primary" anymore. How would this help?
There's no more requirement for it to be unique as far as I can see.
There is no unicity check anymore; I just kept the first one special because some private code at GitHub seems to rely on the primary_extension property for the Gist editor. As I can't hack this (it is private), I can't remove it entirely.