Duplicate function parameters are accepted

### Project version

master (released)

### Project

ast

### What happened?

A function may declare the same parameter name twice. Nothing is reported, and the second declaration silently shadows the first.

### Minimal reproduction steps

This compiles, but should not:

``` rust
fn foo(x: u32, x: u32) -> u32 {
    x
}

fn main() {
    let x: u32 = foo(1, 2);
}
```