Now Abraham moved on from there into the region of the Negev and lived between Kadesh and Shur. For a while he stayed in Gerar, and there Abraham said of his wife Sarah, “She is my sister.” Then Abimelech king of Gerar sent for Sarah and took her.  Genesis 20:1-2

Now there was a famine in the land—besides the earlier famine of Abraham’s time—and Isaac went to Abimelech king of the Philistines in Gerar. Genesis 26:1

Abraham traveled to Gerar and King Abimelech saw his wife Sarah and desired her.  Abraham said, "She's my sister" and Abimelech took her as his wife.  Before he touched her, Abimelech received a dream from the Lord that said "hands off" so he returned Sarah and gave Abraham a bunch of livestock.  They made a treaty never to war against each other.

Isaac traveled to the same area with his wife Rebekah and told everyone "She's my sister."  Abimelech looked out the window and saw Isaac making out with Rebekah.  Abimelech inquired, "Why did you tell everyone she's your sister?"  Isaac said he was afraid.  Abimelech tells everyone to keep their hands off her.

The stories are different, yet similar.

It could be a different king, like the son of the original Abimelech, so he was unaware of the previous incident.

If it was the same king, Abimelech seemed to have learned his lesson.  No more taking men's sisters until he's checked it out first.

Isaac was his father's son.  The apple did not fall far from the tree.  He probably knew of his father's deception and tried it out himself.  His dad did make a lot of money off this incident.