Feedback: An item with the same key has already been added.

If you add the same item twice to a `paket.references` file you get:

```
Locked version resolutions written to \paket.lock
Paket failed with:
        An item with the same key has already been added.
```

I would suggest that paket should at least tell me which file is invalid :). 
The error message itself could also be improved.
This could be a bug as well.
