Controlled Checkbox array example

[leesei]

When checking "A", "C", the controlled field will return ["a",null,"c"] upon submit.
https://react-hook-form.com/docs/usecontroller

Code Sandbox:
https://codesandbox.io/s/usecontroller-checkboxes-99ld4?file=/src/App.js

I improved the example using Set:
https://codesandbox.io/s/usecontroller-checkboxes-forked-dfxcln?file=/src/App.js

Background issue:
react-hook-form/react-hook-form#476

I'm not sure whether the original behavior is intentional. At least it doesn't match the behavior of uncontrolled component.
Shall Set be adopted?