Extending ReferenceManyField with new type cause dupes choices to appear

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Hi!
I managed to extend `ReferenceManyField` like this :

```
import ReferenceManyField from 'admin-config/lib/Field/ReferenceManyField';
class ReferenceManyCustomField extends ReferenceManyField {
constructor(name) {
super(name);
this._type = 'reference_many_custom';
}
}
export default ReferenceManyCustomField;
```

But now the `choices` displayed include the current field value.
If I remove the line where I change the `_type` property, it goes back to normal.
Any hints on why this is happening?

Start in admin-config/lib/Field/ReferenceManyField and compare the behavior with and without changing _type to reference_many_custom. Reproduce the custom subclass case and verify that the choices no longer include the current field value when the type is extended.

Written by the indexing model from the issue text.