I think if they see the game on Steam they could buy it there without a key, it's probably just us germany here and another country I keep forgetting. Might have been China I don't know.
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Actually it's pretty normal to expect games that I buy elsewhere which then become available on Steam, to be able to link the existing purchase there and own it on Steam as well. Some games are not done this way however, which is why I asked. That's fine though, I don't mind waiting to see it on Steam.