consider throwing if a single component pattern has duplicate named groups

[wanderview]

Currently, its possible to do this:

let p = new URLPattern({ pathname: '/:id/:id' });
let r = p.exec({ pathname: '/foo/bar' });

console.log(r.pathname.groups.id); // logs 'bar'

I think this behavior is a bit surprising and probably not what people would expect. Its possible we could support this with some different behaviors (require groups to match the same value, create a sublist of values, etc), but for now I'd like to throw on duplicate names to prevent folks from depending on this unintended behavior.