Gson does not detect duplicate keys in the nested json

Gson is unable detect duplicate keys in the nested json

See below code snippet

```
public static void main(String[] args) throws IOException {
Map map;
//this does not detect duplicate 'key'
map = new Gson().fromJson("{'a':'x','aa':{'key':'x','key':'y'}}", Map.class);

//As expected , this call fails with Exception in thread "main" com.google.gson.JsonSyntaxException: duplicate key: a
map = new Gson().fromJson("{'a':'x','a':'y'}", Map.class);
}
```

Is this the expected behavior ?
If so is there any alternative to fail in the first case as well?