This is a configurable option. If you go to the "Reward" section of the campaign settings, you'll see"I would like to reward campaign participants - each time when five of their friends convert on my website". "Each time" is clickable and can be changed to once. When set to "each time" they'll be automatically given the reward for each time they reach 10 referrals. When set to once, they'll only be rewarded once when they reach 10 referrals.