Duplicating names in a type parameter should be illegal.

The following function compiles:

```
fn foo<A: Share, A>(a: A) -> A{ a }
```

When unused the compiler issues the standard unused code warning. 

As far as I can tell, there is no way to actually use the function without annotating the types for everything (`foo::<&str, uint>(5u)`) because otherwise the compiler can't infer the type of the function.
