Pick tasks will get failed if the item & quantity process in the Pick Task twice.
You may use the Saved Search configuration above to determine any duplicating Pick Task done for a single transaction.
Refrain on processing a wave transaction with the same item and quantity. However if you have a Sales Order for an item with 5 quantities you may create a wave for item with 2 quantities and another wave for item with 3 quantities. In total you will have a 2 wave for an item with 5 quantities. If an item with 2 quantities is recorded in 2 waves for item with 2 quantities each making the total item to 4 quantities instead of 2 only will cause the pick task fail.