Validate Duplication on two fields

Achieved by using this script.
But when i create form and after saving it i edit the form its validate me to edit anything because the record already exist with the same employee and same date.
seems like trigger issue, i just want this function to run while creating form.
can anyone point out that where am i wrong?

frappe.ui.form.on("Time Tracking", "validate", function(frm) {

frappe.call({
        method: "frappe.client.get_value",
        args: {
                doctype: "Time Tracking",
                fieldname: "name",
                filters: {
                        employee: frm.doc.employee,
			period_start_date: frm.doc.period_start_date,
			period_end_date: frm.doc.period_end_date
                }
        },
        callback: function(response) {
             var employee = response.message;
             if (employee) {
                  frappe.msgprint("Same Employee with the Same Time Period is Already Exist in Record.");
			validated=false;
    			return false;
		
             }
        }
});
});