Hi Oliver! I agree that this is more of a survey strategy problem.
When you do the baseline survey, you need to collect as much identifying information as possible. So maybe it's a participant name, father's name, mother's name, children's names, birth date, etc. That information combined with the location is usually enough to identify the participant.
I've seen other campaigns where they use a national ID or leave an ID card for the person to keep. You can also take photos of participants, but then you are generating a lot of data that you might not use.