As part of our incentive limits feature, we collect and store each email your recipients use to redeem their rewards.
We then cross-reference each recipient email against all other emails they might have used in previous incentive batches when they redeemed a reward.
In other words, if someone uses email@example.com to get paid via PayPal as part of an Ethnio reward, but used firstname.lastname@example.org to participate in your study, and then goes back a month later using email@example.com, but then when it's time to get paid again, uses the same PayPal email of firstname.lastname@example.org, we associate them all back to the same person and show you the limit. This gives Ethnio unprecedented view into preventing professional survey-takers from gaming your studies, because almost nobody has multiple PayPal or Amazon accounts, but people frequently use multiple email addresses to try and qualify for surveys and studies. It's an entire industry.