A back-up offer is an offer submitted to a seller knowing he/she has already accepted another offer.  It is a secondary offer.  Sometimes the seller accepts the back-up offer contingent on the prior sales transaction by the primary purchaser to fail within a specified period  

If the primary purchase contract defaults for any reason, the seller must make sure that the first offer is terminated in writing.  Otherwise, the seller may be contractually forced to sell the same property to two different buyers. The best practice is to obtain a release from the first buyer and the listing agent should help the seller complete the necessary legal forms to terminate the first contract.