_Follow along with the video_ --- ### Tracking To prevent users from claiming their tokens multiple times and draining the contract's funds, we need a mechanism to track who has already claimed their tokens and stop them from doing so again. We can declare a mapping of addresses to boolean values to keep track of whether an address has claimed its tokens: ```solidity mapping(address => bool) private s_hasClaimed; ``` We then update this value in the `claim` function: ```solidity //.. if (!MerkleProof.verify(merkleProof, i_merkleRoot, leaf)) { revert MerkleAirdrop__InvalidProof(); } s_hasClaimed[account] = true; emit Claimed(account, amount); //.. ``` > 👮‍♂️ **BEST PRACTICE** > Ensure you follow the checks-effects-interactions (CEI) pattern to prevent vulnerabilities such as re-entrancy attacks. ### Verifying At the beginning of our contract, we can implement a check to verify if an address has already claimed: ```solidity if (s_hasClaimed[account]) { revert MerkleAirdrop__AlreadyClaimed(); } ```