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

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

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Content Screens

Home Page Cards
Category, Course, Trainer pages
Improve Activity And Play Tabs
Top level navigation