I'm currently reading 1 Samuel and yesterday I read Chapter 16 and today I read Chapter 17 and something caught my attention. 1 Samuel 16:14-23 NIV 14 Now the Spirit of the Lord had departed from Saul, and an evil spirit from the Lord tormented him. 15 Saul’s attendants said to him, “See, an evil spirit from God is tormenting you. 16 Let our lord command his servants here to search for someone who can play the lyre. He will play when the evil spirit from God comes on you, and you will feel better.” 17 So Saul said to his attendants, “Find someone who plays well and bring him to me.” 18 One of the servants answered, “I have seen a son of Jesse of Bethlehem who knows how to play the lyre. He is a brave man and a warrior. He speaks well and is a fine-looking man. And the Lord is with him.” 19 Then Saul sent messengers to Jesse and said, “Send me your son David, who is with the sheep.” 20 So Jesse took a donkey loaded with bread, a skin of wine and a young goat and sent them with his son David to Saul. 21 David came to Saul and entered his service. Saul liked him very much, and David became one of his armor-bearers. 22 Then Saul sent word to Jesse, saying, “Allow David to remain in my service, for I am pleased with him.” 23 Whenever the spirit from God came on Saul, David would take up his lyre and play. Then relief would come to Saul; he would feel better, and the evil spirit would leave him. 1 Samuel 17:55-58 NIV 55 As Saul watched David going out to meet the Philistine, he said to Abner, commander of the army, “Abner, whose son is that young man?” Abner replied, “As surely as you live, Your Majesty, I don’t know.” 56 The king said, “Find out whose son this young man is.” 57 As soon as David returned from killing the Philistine, Abner took him and brought him before Saul, with David still holding the Philistine’s head. 58 “Whose son are you, young man?” Saul asked him. David said, “I am the son of your servant Jesse of Bethlehem.” I know I'm pretty forgetful myself, but I don't think I'd forget someone like David, who played an instrument that would bring peace to me in my distress.
Kind of a wild guess, but I wonder how much of that style of writing comes down the scribes themselves. There is probably some ancient style that is intended to reinforce a point about someone or something. I'd have to defer to my betters in this regard.
Or perhaps Saul, who we know was enamored with his own pomp and position, had so many Servants that he couldn't remember them all
In 1 Samuel 17 Saul isn't asking who David is, but who David's father is. Given that Saul was tormented in 1 Samuel 16, I doubt he remembered the details of what his servant told him about David, who evidently wasn't Abner, as he didn't know the details either.
This also seems like a reasonable reason. The ESV Study Bible notes agrees with you too. 1 Sam. 17:55–58 whose son is this youth? Even though Saul knew David from before, he would not remember the name of David’s father. Saul is asking about David’s background—his family and hence his social status or pedigree—so that he may ask his father to let him keep David permanently (see 18:2).