Aang has used ice before. He is able to redirect and manipulate lightning so that counts. Never seen him metalbend but he might have picked up the skill somewhere in his life. The secondary abilities for water is blood bending,which he can resist, and healing( which one needs to be born with the ability.)
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He couldn't resist blood bending because he has the ability, Katara could fight being blood bent by being a stronger bender without having the skill her self.just like Amon was a stronger bender than his brother. Aang used the Avatar state to over power the bloodbending. I will admit though after rewatching the last airbender he does use ice but very rarely.
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Technically he's born with it all
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Not really. He doesn't get his past lives' abilities unless he's in the avatar state. Without it it's all him.
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He's still born with the ability to do it, he just has to learn how to use it. So yeah he's born with it.