It does appear that David had already been introduced to Saul where he had become Saul’s music therapist. Its surprising that these verses appear to be introducing him again. David is shown as coming from home where he left the sheep with a keeper so he could do his fathers command. In this verse, we witnesses to David’s diligence in keeping the sheep. He found an assistant while he was away. Later in v.55, Saul asks the question: whose son is this young man? This is what one scholar says, ” There are several possible explanations for why Saul asks about David in chapter 17 after he had been introduced to him in chapter 16. As a preface, we should note that Saul was not inquiring of the identity of David per se in chapter 17; rather, he was asking who David’s father was (verses 55, 56, and 58). We assume he knew David’s name. David’s answer, in verse 58, is, “I am the son of your servant Jesse of Bethlehem.” Whatever the case, it takes time to introduce any champion in a story. We do not have the space. Know this, God is creating every possible scenarios to introduce you. Your faith is one, your skill is another, your victory yet another. They will all combine to bring you into your announcement. Nothing is lost. You may not understand it all now, but the plots will dovetail into a beautiful tapestry of your life.
And David rose up early in the morning, and left the sheep with a keeper, and took, and went, as Jesse had commanded him; and he came to the trench, as the host was going forth to the fight, and shouted for the battle. 1 Samuel 17:20 KJV