For certain activities, you unlock Achievement Badges. Some of these are Inner Circle, Sharpshooter, Big Spender, Overachiever, Overtime, Friendlt, Wildfire and Champion.

Each badge has 3 levels that you progressively unlock.

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Duolingo

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

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Gamification

Daily check ins
Gamification badges
Gamification: Streak, Lives and Gems
Competitions and gamification