title: Salesforce apex trigger duplicate id in list
body: <p>In my order insert trigger, i need to update contact for the order information, I know there are duplicate records in the my contact list. so I use
  Contact[] contactToUpdate = new List(new Set(contactList));
  to remove the duplicates. </p>
  
  <p>but I still receive the error message: duplicate in List. </p>
  
  <p>any idea?? </p>
  
  <p>Thanks</p>