Not a bug. I can see from using https://okean123.github.io/NFTExplorer/home.html that you own that card. You probably won it after completing a mission at some point.
This post gets you another entry into the weekly prize draw to win a 3 card booster pack.
Good luck!
!BEER
Hey, you are right. I didn't even notice that I got another booster.
Thanks :)