If you want to make sure that past respondents or participants in a given screener aren't able to book another session, Ethnio has a default setting to prevent duplicates using email address. If you want to allow duplicates, head to scheduling >> settings >> Email Rules for Participants and change allow previous respondents... to Yes. You'll now allow duplicates from that point forward, but any past duplicates will still be blocked, although you can just change their status to new in order to invite a duplicate.
