1 Samuel 18:21

And Saul said, I will give him her that she may be a snare to him, serve as a bait or lure to him, and that the hand of the Philistines may be against him; he had a scheme in mind which would surely be successful. Wherefore Saul said to David, Thou shalt this day be my son-in-law in the one of the twain, literally, "The second time thou shalt become my son-in-law," namely, first by the betrothal to Merab, the second time by the actual marriage to Michal.