At Level 21 I clicked on the button 'challenge Gate Keeper'. When my pet arrived to the battle there was the Three angry bears again I had already at level 20.

I have this happen sometimes. I think it has to do with double clicking after or before a battle, it makes it think like you've still been in battle with the previous challenger.

Thank you. I stopped with the battle quests at this point for today, if there is an issue.

I haver arthritis and seizures and I can confirm that this is true. Double (or in my case triple or quadruple) clicking results in the same challenger going for the next few times. It wasn't considered an exploit last time this happened, so I think you are okay.

I'm not even sure that they fixed/wanted to fix this when it happened awhile ago, so this may still be a thing from back then.

I have Chrome OS and Browser, Lenovo ThinkCentre