Works for me:

User has two preferred English languages (US and UK for example). osLanguages are [en,de,tr,en] for him and will be translated to [de,tr,en] because first EN already exists in suggested languages list and will not be added to the list of languages again. I thought that it is not an issue (who uses two English?) but app uses first language from list as primary language, so user will have Turkish as primary language instead of English according to preferred languages from iOS Settings.

https://github.com/wikimedia/wikipedia-ios/pull/1581