There has been reports that when trying to purchase expansion DLC such as Ardennes Assault or The Western Front Armies, an error message pops up saying that you already own this item. This is due to Steam not recognizing the DLC as a separate entity to the base game. To get around this, purchase the DLC as a gift and then enter the Steam key that you receive into your Steam Library. After doing this, you should be able to access the DLC without issue.
If you are still experiencing a problem, please contact Support at https://support.sega.co.uk/hc/en-us/requests/new