Using an interface for two imports causes an error

```wit
interface iface {
f: func()
}

default world w {
import p1: self.iface
import p2: self.iface
}
```

This produces the following error:

```
$ wit-bindgen markdown example.wit
Error: interface `p2` imported more than once
--> example.wit:7:9
|
7 | import p2: self.iface
|
```

Is it actually not allowed to have two imports with the same interface? That seems surprising to me.