Because wallets are free, one person can look like a thousand users. Sybil resistance is the problem every permissionless distribution has to solve, whether for an airdrop, a vote, or a reputation system.
Detection relies on clustering: wallets funded from one source, transacting at the same times, following identical paths. Projects publish exclusion lists after analysis, and excluded wallets usually have no appeal.
In governance the same attack is cheaper to defend, because voting power is weighted by tokens rather than by identity. That converts the problem into a different one: plutocracy, where whoever holds the most decides. See governance-quorum.
Related: airdrop-farming, airdrop, governance-quorum, dao