Shatter the Source

Convoke is a static ability that functions while the spell with convoke is on the stack. “Convoke” means “For each colored mana in this spell’s total cost, you may tap an untapped creature of that color you control rather than pay that mana. For each generic mana in this spell’s total cost, you may tap an untapped creature you control rather than pay that mana.”

The convoke ability isn’t an additional or alternative cost and applies only after the total cost of the spell with convoke is determined.

A creature tapped to pay for mana in a spell’s total cost this way is said to have “convoked” that spell.

Multiple instances of convoke on the same spell are redundant.