hi

i have checked ur code is correct run in background script

var dupRecords = []; 
 var gaDupCheck1 = new GlideAggregate('cmdb_ci_computer'); 
 gaDupCheck1.addQuery('active','true'); 
 gaDupCheck1.addAggregate('COUNT', 'serial_number'); 
 gaDupCheck1.groupBy('serial_number'); 
 gaDupCheck1.addHaving('COUNT', '>', 1); 
 gaDupCheck1.query(); 
 while (gaDupCheck1.next()) { 
 dupRecords.push(gaDupCheck1.serial_number.toString()); 
 } 
 gs.print(dupRecords);

it's giving the value 

56WHL71,AEX5635Z0001AC,L3BB911

only mistake this line

javascript : new script_include_name.method_name();

thanq