In 1 Samuel 18, shortly after David kills Goliath, King Saul has started feeling envious and afraid of David. It says the reason he was afraid of him was that God had abandoned Saul and was now with David instead. Then Saul hears that his daughter Michal is in love with David. This pleases him, because he can use it as an opportunity to try to get David killed. So Saul sends David off to fight the Philistines, to prove that he’s worthy to marry the princess.
It doesn’t go as Saul planned. Instead of getting killed, David succeeds in killing twice as many Philistines as Saul had challenged him to kill, so Saul has to let him marry his daughter. Then Saul suddenly realizes that God is with David and that Michal is in love with David… again? And that makes him even more afraid of him. But how can Saul be realizing those things just now, if he already knew them?
Maybe Saul just forgot. That would mean he would have to be forgetting about things that were significant to him awfully quickly. But it wouldn’t be the first time (or the last time) Saul inexplicably seemed to quickly forget something important. For instance, he apparently forgot that David had already earned the right to marry the king’s daughter when he killed Goliath. Saul’s son Jonathan also has some baffling episodes of forgetfulness. It must run in the family.
The last part does not refer to the same realization that had already been mentioned, since Saul’s reaction is different this time. The first time he realized that Michal loved David, Saul was pleased; the second time, he was afraid. The first time he realized God was with David, Saul became afraid of David for the first time; the second time he realized it, he became more afraid.