Slitherine

I bought the Kursk expansion from the Matrix Games website. I then registered the serial number on Slitherine's page to generate a Steam key per the instructions. Steam is balking at this saying it's a Duplicate product code- that it's already been activated on another Steam account (I only have one Steam account).
Could someone please help me out with this? Thank you!

sorry for your issue

contact support[at]slitherine.co.uk with your order details and they will help, but not back until the 4th