    function _transfer(address from, address to, uint256 amount) internal override {
        if (from == address(0)) {
            revert Errors.ERC20_TRANSFER_FROM_THE_ZERO_ADDRESS();
        }
        if (to == address(0)) {
            revert Errors.ERC20_TRANSER_TO_THE_ZERO_ADDRESS();
        }
        _refreshCumulativeYield();
        _refreshYield();

        uint256 startingFromBalance = this.balanceOf(from);
        if (startingFromBalance < amount) {
            revert Errors.ERC20_TRANSFER_AMOUNT_EXCEEDS_BALANCE();
        }
        uint256 newFromBalance = startingFromBalance - amount;
        uint256 newToBalance = this.balanceOf(to) + amount;

        uint256 previousEpochCumulativeYield_ = _previousEpochCumulativeYield;
        uint256 newFromBaseBalance = WadRayMath.wadToRay(newFromBalance).rayDiv(previousEpochCumulativeYield_);
        uint256 newToBaseBalance = WadRayMath.wadToRay(newToBalance).rayDiv(previousEpochCumulativeYield_);

        if (amount > 0) {
            _totalBaseSupply -= (_baseBalances[from] - newFromBaseBalance);
            _totalBaseSupply += (newToBaseBalance - _baseBalances[to]);
            _baseBalances[from] = newFromBaseBalance;
            _baseBalances[to] = newToBaseBalance;//<--------if from==to,this place Will overwrite the reduction above
        }

        emit Transfer(from, to, amount);
    }
    //test from == to
    function test_transfer_same() public {
        _KIBToken.mint(_alice, 10 ether);
        assertEq(_KIBToken.balanceOf(_alice), 10 ether);
        vm.prank(_alice);
        _KIBToken.transfer(_alice, 5 ether);   //<-----alice transfer to alice 
        assertEq(_KIBToken.balanceOf(_alice), 15 ether); //<-----increases 5 eth
    }
forge test --match test_transfer_same -vvv

Running 1 test for test/kuma-protocol/kib-token/KIBToken.transfer.t.sol:KIBTokenTransfer
[PASS] test_transfer_same() (gas: 184320)
Test result: ok. 1 passed; 0 failed; finished in 24.67ms
balance[to]-=amount
balance[from]+=amount
    function _transfer(address from, address to, uint256 amount) internal override {
        if (from == address(0)) {
            revert Errors.ERC20_TRANSFER_FROM_THE_ZERO_ADDRESS();
        }
        if (to == address(0)) {
            revert Errors.ERC20_TRANSER_TO_THE_ZERO_ADDRESS();
        }
        _refreshCumulativeYield();
        _refreshYield();

+       if (from != to) {
	        uint256 startingFromBalance = this.balanceOf(from);
	        if (startingFromBalance < amount) {
	            revert Errors.ERC20_TRANSFER_AMOUNT_EXCEEDS_BALANCE();
	        }
	        uint256 newFromBalance = startingFromBalance - amount;
	        uint256 newToBalance = this.balanceOf(to) + amount;

	        uint256 previousEpochCumulativeYield_ = _previousEpochCumulativeYield;
	        uint256 newFromBaseBalance = WadRayMath.wadToRay(newFromBalance).rayDiv(previousEpochCumulativeYield_);
	        uint256 newToBaseBalance = WadRayMath.wadToRay(newToBalance).rayDiv(previousEpochCumulativeYield_);

	        if (amount > 0) {
	            _totalBaseSupply -= (_baseBalances[from] - newFromBaseBalance);
	            _totalBaseSupply += (newToBaseBalance - _baseBalances[to]);
	            _baseBalances[from] = newFromBaseBalance;
	            _baseBalances[to] = newToBaseBalance;
	        }
+       }
        emit Transfer(from, to, amount);
    }
