Having duplicate relaxed default bounds is unsupported.

Erroneous code example:

```compile_fail,E0203
struct Bad<T: ?Sized + ?Sized>{
    inner: T,
}
```

Here the type parameter `T` cannot have duplicate relaxed bounds for default
trait `Sized`. This can be fixed by only using one relaxed bound:

```
struct Good<T: ?Sized>{
    inner: T
}
```
