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

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

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Gamification

Upgrade subscription
Redeeming reward
Gamification
Daily check ins