It is very simple. We asked in the airdrop questionnaire to specify Username on our platform. It is impossible to receive it without registration and confirmation by mail. After Airdrop, we compared the emails that are listed in the questionnaire with the email addresses of the site database.
Most of the list fell on this one. They just entered their data in the questionnaire.
In addition, we tested js scripts to detect multi-accounts by cookies, ip, user-agent, traffic source and some other parameters. The script read all these parameters, compared them and added antifraud points. If the account is suspicious, then the tasks performed by him are checked manually. We did this because we are interested in real participants and not in bots.