Claim Airdrop doesn't add another claim reward value to existing value, how the value will change but will add to existing reward?

source code link:
This contract send token to address/addresses, and the user will redeem the airdrop token of this contract

PROBLEM:After adding or airdropping to multiple address using this function:

function addSocial(address[] _users, uint256[] _tokens) onlyOwner public {
    require(_users.length > 0 && _users.length == _tokens.length);
    for (uint i = 0; i < _users.length; i++) {
      social[_users[i]] = _tokens[i];

If user did not claim the token yet, And owner wants to add another airdop rewards, token rewards value of user will change instead of adding the new rewards to existing reward of an user. How to retain the value of previous airdop?

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code here

its a question mate, trying to figure out the process of airdropping of an address multiple times, and can claim multiple time.