When you scan the SecTrace pass, a few selected details of the data are visible for verification. For example, after the scan you will see the initials, the entered zip code and the last 3 digits of the phone number. This data is intentionally not printed on the SecTrace passport.
So you can ask your guest for one of these 3 details to make sure that he uses the right passport. (The rest of the data is of course encrypted and cannot be read out to ensure privacy).
Also, the SecTrace pass itself shows the name and date of birth of the owner – so that a mix-up can easily be avoided.