Samsung Galaxy were the 2017 World Champions, beating out SK telecom T1 in a decisive 3-0 victory. As with all World Champions, the team will be able to decide which champion they would like to receive a skin.
Typically, the champions that receive skins are the champions the players used in a match in the Finals. As of now, there is no champion who has received two championship skins.
But that all might change when Samsung Galaxy's Kang "Ambition" Chan-yong chooses his champion for a skin. In an interview with InvenGlobal, Ambition stated that he wanted Jarvan IV to be his skin despite Jarvan IV already having a World Champion skin from the Season 1 World Championship.
Ambition played Jarvan IV really well against SK telecom T1 in the matches, and especially in Game 2 when he landed a nasty combo which decided the game for them.
Riot Games does not really have a policy regarding a champion getting two World Championship skins, but they do enjoy if a new champion gets one.
Hopefully, Ambition will be able to get his Jarvan IV skin because he really deserves it after the way he played at Worlds.