Starting from version 6.20.08.03 there is a new setting that changes the behavior of the lookup controls in the (smart)kiosk. This new setting allows you to prevent making duplicate profiles with the same e-mail, phone number or whatever has been configured.
The name of the setting is “CAN CREATE DUPLICATES ON LOOKUP”, the group is kiosk.
Create duplicates on true (default state) in (Smart) kiosk
Create duplicates set to false