What is the benefit from sending 0 eth? I think they should have an eth address with transactions on it at a specific date. This way you make sure it is not newly created to join the airdrop.
hmm... maybe handle Airdrop via a mobile app? this way you could exclude lots of multi signups.
Sending 0ETH can reduce your workload.
It's my point，it can make a smart contract automatically refunded token.