It happened, when Saul was returned from following the Philistines, that it was told him, saying, Behold, David is in the wilderness of En Gedi.
It is not any more incredible that David spared Saul's life twice than that he spared it once.  We hold both accounts to be absolutely true as recorded.  The events are so different that there is no intelligent device by which one can understand them as variable accounts of only one incident.