AThe multiple citizenship holders holding Korean citizenship are not allowed to exercise the foreign nationality within Korea. Therefore, they are required to present their Korean passport for immigration inspection when they enter or exit the Republic of Korea and they do not need to apply for a K-ETA.
However, they need to present both the Korean passport and the alternate country’s passport to obtain the boarding pass.
If the personal information (name, date of birth, etc) in the two passports does not match, the airline may refuse to issue the boarding pass for not having a valid K-ETA as per their policy. In that case, they may need to apply for a K-ETA even though they have a Korean passport. Therefore, if the personal information in the two passport does not match, please check this matter with the airline in advance.
Meanwhile, those who do not have a Korean passport need to apply for it at the nearest Embassy or Consulate of the Republic of Korea and enter Korea with the passport.