David appears to be introduced to Saul twice. First in 1 Samuel 16 as a lyre player to sooth Sauls distress when the evil spirit came upon him. And "Saul loved him (David) greatly, and he became his armour bearer." The second introduction occurs when David sallied forth against Goliath. Saul asked Abner his commander, "Whose son is the lad, Abner? And Abner said, "By your life king, I do not know." 1 Saul 17.

Thank you for your question. There are quite a few answers to this question. Saul obviously met David earlier (as mentioned in Chapter 17:21). So why did Saul ask Avner at a later time (17:55): "Who is this lad?", referring to David. I'll mention a few answers: 1. Rashi: Saul knew who David was but asked what his "yichus", or pedigree, was. He wanted to know if he was descended from Peretz, the son of Yehuda. If so, he definitely would be king. 2. Metzudat Dovid: Saul forgot who David was because of the bad spirit that came upon him. 3. Malbim: a G-dly spirit came upon David and Saul couldn't recognize him anymore. For further explanations see Radak, Mahari k'ra and the Malbim's second explanation. All the best