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

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

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Gamification

Competitions and gamification
Card Gamification: Behaviour Cards
Gamification
Converting steps to coins