If the contract is reused with another Merkle root and there is a user participated in the previous airdrop, user's claimed amount will not be set to 0. This means if the new total amount is 1000 and the user has claimed amount (from previous aidrop) equals to 500, the user will be able to receive only 500 tokens instead of intended 1000. The `set_merkle_root` is called only by the owner who is trusted. Therefore, we can assume that the owner will not call the function during the existing vesting period or before all users claim their token amounts. The owner can check manually if all users have claimed their tokens.