A Letter of Follow Up or Duplicate Order is a document that is used to follow up on an order that has already been placed, either to check the status of the order, or to re-order additional items. The letter can be sent by either the customer or the seller, depending on the situation. Types of Letter of Follow Up or Duplicate Order: 1. Follow-Up Letter: A follow-up letter is sent to check the status of an order that has already been placed, either to confirm that the order is still on track, or to inquire as to why it has not yet been filled. 2. Duplicate Order Letter: A duplicate order letter is sent when the customer needs to re-order additional items, either because the first order was incomplete or because additional items are needed for an upcoming event.