reject AOs with duplicate parameter names

[michaelficarra]

I introduced an error into my spec where I had an AO with more than one parameter with the same name, but ecmarkup didn't complain about it. The build should have failed.

[bakkot]

I'd've expected #679 to handle this. Were you running latest?

[michaelficarra]

The version released a few hours ago? No. We haven't yet bumped it in 262. If it's recently been fixed, that's cool. Do we have a test specifically for duplicate parameter names? If not, we should add one and then we can close this.