Referral schemes are retailer specific - this means you cannot refer someone at the same retailer multiple times. However a person can be referred multiple times with the same Yoyo account*
In short, this means that you can refer friends at several retailers within the Yoyo app or you can refer friends to another Yoyo-powered app (i.e. Caffè Nero, Harris + Hoole, SOHO Coffee Co.).
*Note that Yoyo Wallet powers the Caffè Nero app, the Harris + Hoole app, and the SOHO Coffee Co. app. The same Yoyo account will be used across all apps.