OK... Lets say you didn't know that you already own the same DLC due to huge lib and then you revealed the key... once you tried to activate it you realized that you already own the same app/DLC or you didn't because at that moment you were logged in your alt-account (but that's also your problem, it literally says:"Please remember, all g..." at the won page) And then what? You kept the key for later use or gave away on another platform.
You could have asked for re-roll (i assume you tried to activate the key on your main account so the key was in safe condition)
Either way it is your fault. You should have read the rules.