UnixFS duplicate paths during import

Following on from https://github.com/ipfs/js-ipfs/issues/2622

You can have the situation where a user tries to build a UnixFS directory with duplicate paths:

```javascript
await ipfs.add([{
path: 'folder/file',
content: Buffer.from('hello world')
}, {
path: 'folder/file',
content: Buffer.from('another hello world')
}])
```

Current behaviour is last-file-wins for both `go-IPFS` and `js-IPFS`.

Should this be the case or should they error? The API makes it trivial to do this kind of operation, but file systems typically do not support entries with duplicate names.

Once there is an agreement, this should be documented in the spec order for DAGs to be recreated without reference to `go-IPFS` or `js-IPFS`.