Clubs are a social area where you can do certain challenges and interact with the community. As of early May 2019, the feature is being A/B tested against Leaderboards and might be phased out. Users are making petitions to save the feature.

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Duolingo

Onboarding
Top level tabs
Learning experience
Gamification: Badges
App icon change
App Store screenshots
Duolingo Plus
Change or add language
2019 redesign
Create profile
Asking for notiification permissions during onboarding
Gamification memes
Gamification: Streak, Lives and Gems

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Content Screens

Product Page
Top level tabs
Car detail page
Restaurant page