[2020] When I try to register my serial key on Factorio's website, it tells me that the key is already in use.

Bought the game on GOG. When I went to register it on Factorio's website, it told me the key was already in use. Now GOG is telling me Factorio's support has to fix the issue while Factorio's support is telling me GOG has to fix the issue.

Is there anyone I can contact to have this fixed? At first I thought this would be easy to resolve, so I played in single player, but I bought the game for multiplayer and that's locked off...