7.0.0-dev.2: too strict in duplicate label detection

```reason
type t = {
  [@bs.as "a"]
  a: float,
};
```

gives the error

```
  Two labels are named a
```

which is not correct.

(I know, the `bs.as` attribute is superfluous here, but this situation can occur easily in generated code. I saw it in the MaterialUI bindings.)