When I get angry, you buy me silver or gold.
When a person gets cavities or otherwise damages their teeth, they often get silver or gold replacements.
You don't usually see this, but my place is twofold.
There are two sets of teeth (baby teeth and adult teeth). This could also refer to there being teeth on the top of the mouth or the bottom of the mouth, but I'm guessing it's referring to the former.
You often help me, but I can only cause pain.
People brush their teeth to protect them from plaque and cavities. But people often only think about their teeth when they get toothaches.
I'm always destroying things, again and again.
That's the job of teeth... to destroy food :)
You'll lose me once, and sleep through my magic.
Children lose their first set of teeth, and then receive gifts from the Tooth Fairy as they sleep.
But lose me again, and it is far more tragic.
Unfortunately adult teeth don't grow back (without very expensive and painful medical procedures, anyways).