Albert Barnes Commentary 1 Samuel 18:21

"And Saul said, I will give him her, that she may be a snare to him, and that the hand of the Philistines may be against him. Wherefore Saul said to David, Thou shalt this day be my son-in-law a second time." — 1 Samuel 18:21 (ASV)

In the one of the two - Some prefer “the second time” (Job 33:14). The first contract had been broken by giving Merab to Adriel.