# [Retina 0.8.2], 29 bytes

```
^
aaabbbbac
O`c|..
cc|(..)\1

```

[Try it online!][TIO-loobarov] Link includes test cases. Explanation:

    ^
    aaabbbbac

Add another copy of the whole list.

    O`c|..

Sort the elements.

```
cc|(..)\1

```

Delete duplicates. Fortunately it's not possible to match at an odd position.

[Retina 0.8.2]: https://github.com/m-ender/retina/wiki/The-Language/a950ad7d925ec9316e3e2fb2cf5d49fd15d23e3d
[TIO-loobarov]: https://tio.run/##LU4xCoBADNvzD0GXA1/h6AdEzAUHFwdx9O9nWi2lIW2a9trv42Tr@mlrK0hWB4V501MKpKcvZVhGtKaYsOIDQdY6YUYFVrlPZP25Z2D2a2BsWq@442PpgGTGcEuDj@dG/sMX "Retina 0.8.2 – Try It Online"