Can you add a metamagic feat more than once to any particular spell? For example, double Maximize Spell, or
double Extend Spell?
You can apply most metamagic feats more than once. Just stack up the costs, and remember to apply the additional effects
to the basic spell. For example, if you extend a spell twice, you get 3 times the duration, not 4 times the duration. (Each extension adds 100% of the spell's base duration.)
There are a few metamagic feats that are constructed so as to make stacking worthless or pointless. You cannot, for example, get more than maximum damage out of a spell by maximizing the spell more than once. (If you want to send the spell's damage through the roof, use Empower Spell multiple times.) Heighten Spell already allows you set the spell's effective level anywhere you want (and can manage), so there's no point in applying the feat more than once.