Duplicate account happens when someone who is already banned joins again with another account but with the
same IP adress. Now this could go wrong in a few ways that's true. If we say that the person who was banned and you both have dynamic IP's, which means you get different IP addresses when you reset your modem/router. Then it could be that you just happened to get the IP that the other guy was banned with. This is really rare though.
Ps.: There were 2 minutes between the aimbot ban and your ban, is it possible that someone who just got banned resets his router. In the same time you reset your router, you get his IP and join a blackwonder server? I don't know... Or was it your "little brother" in the same house?
Your account seems legit on the other hand, lvl 10, 350 hours, friends, comments etc. So It's up to
@Benedevil to decide.
https://bans.blackwonder.tf/index.php?p=banlist&searchText=121.239.50.55&Submit=Search