Why does Saul ask who David is, when he had met him earlier (1 Samuel 17:55)? - (Got Questions)
Did Saul Know David Prior to Goliath's Death? - Eric Lyons (Apologetics Press)
Why Did Saul Not Recognize His Harp Player David As The One Who Killed Goliath - (Defencing Inerrancy)
Did Saul Know Who David Was? - (Reformed Wiki)