Thanks allen for the qucik reply,

Wrote the script include and called that in on change client script, seems its not working

Client script:

function onChange(control, oldValue, newValue, isLoading) {
   if (isLoading || newValue == '') {
      return;
   }

	var gaj = new GlideAjax('Vendor Checks'); 
	gaj.addParam('sysparm_name','Vendor');
	gaj.addParam('sysparm_field',newValue);
	gaj.getXML(ajaxResponse);
	function ajaxResponse(response) {
		var answer = response.responseXML.documentElement.getAttribute("answer");
		
		var values = answer.split(',');
		
		if (values.indexOf(newValue) != -1)
		  {
			alert("This value already exists");
		  }
					
		
	}

Script include

Vendor : function()
	{
		var value = this.getParameter('sysparm_field');
		var answers =[];
		var gr = new GlideRecord('supplier_maintenance');
		gr.addQuery('u_vendor','vendor_data');
		gr.query();
		while(gr.next())
			{
				answers.push(gr.value.toString());
			}
		return answers.toString();

	},