try this

getDuplicateAssetforAssignedTo: function() {
var str = [];
var gr = new GlideAggregate('cmdb_ci');
//gr.addAggregate('COUNT', 'serial_Number');
gr.groupBy('serial_Number');
gr.addHaving('COUNT', '>', 1);
gr.query();
while (gr.next()) {
str.push(gr.getValue('serial_Number') + '');
}
return str.toString();

},