# [K (oK)], 9 bytes

**Solution:**

    &/2=#:'.=

[Try it online!][TIO-jxvfmzoj]

**Explanation:**

    &/2=#:'.= / the solution
            = / group
           .  / value
        #:'   / count (length of) each
      2=      / equal to 2?
    &/        / take minimum

[K (oK)]: https://github.com/JohnEarnest/ok
[TIO-jxvfmzoj]: https://tio.run/##y9bNz/7/X03fyFbZSl3P1lzB0FjBUsFIwdAASJiAKLCQiYLl//8A "K (oK) – Try It Online"