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Ok here's the facts:
Ben has played for Canada at youth level so in fifas eyes he is Canadian. However he can still change nationality if he meets the requirements in article 8 of the statutes, which he does, as he hasn't played in a competitive match for Canada. Even if he does play for Canada in a competitive match at youth level he still meets the requirements as he's currently eligible for NZ. However, according to the first paragraph of article 8, a player can only change nationalities ONCE. Thus as soon as he changes to NZ he can't change again, regardless of whether he plays a competitive game or not.