My problem is the last item from PO is always overwrite the first added item. Here my snipped code. thank you in advance

User Event

function beforeLoad(context) {
    // try {
    if (context.type == context.UserEventType.VIEW) {


        var objPORec = context.newRecord;
        var parentPOId = objPORec.id;
        var form = context.form;


        //Add Client Script File
        form.clientScriptModulePath = './po_bill_btn.js'
        form.addButton({
            id: 'custpage_create_child_so',
            label: 'Create Vendor Bill',
            functionName: 'vendorBillFunc("' + parentPOId + '")'
        });
    }
}

clientscript

function init(context) {
    currentRecord = context.currentRecord;

    var items = [{ "item_display": "3247", "quantity": 1 }, { "item_display": "3247", "quantity": 1 }];
    for (var x = 0; x < items.length; x++) {
        addVendorBillLine(items, x);
    }
}

function addVendorBillLine(itemList, x) {
    currentRecord.selectNewLine({
        sublistId: 'item'
    });
    currentRecord.setCurrentSublistText({
        sublistId: 'item',
        fieldId: 'item',