I've just purchased Order of Battle: Red Star DLC in Slitherine store. I would like to redeem a Steam key for my game. I tried to do it the standard way (as many times before) but I'm getting an error message saying "Duplicate product code". I'm pretty sure the game is not yet in my steam account. It seems this steam key already belongs to another account for some reason. Please let me know how to remedy the situation. This is how the process looks like.

I created a new helpdesk ticket under my Slitherine account, but it got closed without any action.
Thanks for help.