Duplicated invalid fragment specifier error

``` rust
macro_rules! mac {
    ($x:xyzzy) => ($x)
}
fn main() {
    mac!(0);
}
```

```
test.rs:2:6: 2:14 error: invalid fragment specifier `xyzzy`
test.rs:2     ($x:xyzzy) => ($x)
               ^~~~~~~~
test.rs:2:6: 2:14 error: invalid fragment specifier `xyzzy`
test.rs:2     ($x:xyzzy) => ($x)
               ^~~~~~~~
```
