Changes.md

// create a new mapping, which will check if the _receiver has already locked funds.
mapping(address => bool) public depositer;

function deposit(uint256 _amount, uint256 _duration, address _receiver) external override {
        if (_amount == 0) {
            revert ZeroAmountError();
        }
        // checks if the _receiver has already locked tokens
        require (depositer[_receiver] == false, "Already locked funds");
        
        // Don't allow locking > maxLockDuration
        uint256 duration = _duration.min(maxLockDuration);
        // Enforce min lockup duration to prevent flash loan or MEV transaction ordering
        duration = duration.max(MIN_LOCK_DURATION);

        depositToken.safeTransferFrom(_msgSender(), address(this), _amount);

        uint256 mintAmount = _amount * getMultiplier(duration) / 1e18;

        depositsOf[_receiver].push(Deposit({
            amount: _amount,
            shareAmount: mintAmount,
            start: uint64(block.timestamp),
            end: uint64(block.timestamp) + uint64(duration)
        }));

        _mint(_receiver, mintAmount);
        
        // makes _receiver's bool as true, so he can't call the function deposit() again till he withdraws the tokens.
        depositer[_receiver] == true;
        
        emit Deposited(_amount, duration, _receiver, _msgSender());
    }
    
    function withdraw(uint256 _depositId, address _receiver) external {
        if (_depositId >= depositsOf[_msgSender()].length) {
            revert NonExistingDepositError();
        }
        Deposit memory userDeposit = depositsOf[_msgSender()][_depositId];
        if (block.timestamp < userDeposit.end) {
            revert TooSoonError();
        }

        // remove Deposit
        depositsOf[_msgSender()][_depositId] = depositsOf[_msgSender()][depositsOf[_msgSender()].length - 1];
        depositsOf[_msgSender()].pop();

        // burn pool shares
        _burn(_msgSender(), userDeposit.shareAmount);
        
        // return tokens
        depositToken.safeTransfer(_receiver, userDeposit.amount);
        
        // makes _receiver's bool as false, so he can call the function deposit() and lock his funds again.
        depositer[_receiver] == false
        
        emit Withdrawn(_depositId, _receiver, _msgSender(), userDeposit.amount);
    }