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? Is there a direct email to support? Thank you!

support[at]slitherine.co.uk, but as it's holiday time, it may take a few days for a reply

Thank you. I just emailed support. Hopefully it will not take too long after the Holiday.

Cheers