Genesis 26:9

And Abimelek called Izhak, and said, Nevertheless she is thy wife; and why hast thou said, She is my sister? And Izhak answered him, Because I said in my heart, Lest they kill me on her account.

ויקרא...אך הנה אשתך, the word אך in this verse means the same as the word אבל, “but, however.” It also appears in this sense in Exodus 31,13 אך את שבתותי תשמורו, “but you must observe My Sabbath days,” (even when engaged in building the Tabernacle) In Exodus 21,22, when speaking about someone dying as a result of an injury deliberately inflicted upon him, the Torah writes אך אם יום או יומים יעמוד, “but if the victim can stand on his feet for a day or two days, etc.” (his death is not directly attributable to the injury described). In our verse, Avimelech challenges Yitzchok who had presented his wife as being his sister by saying: “but I have seen with my own eyes that she is your wife!” He used the fact that he had observed Yitzchok being intimate with Rivkah as proof that therefore she must be his wife, as no one sleeps with a woman who is not his wife.

ואיך אמרת, the Avimelech mentioned in our chapter here was not the same one that lived in the days of Avraham. Had he been the same it is inconceivable that he could have asked Yitzchok such a question. He had, after all, had experience with Sarah and Avraham concerning the same subject. The Kings of the Philistines generally assumed the title “Avimelech,” just as the Kings of Egypt generally assumed the title “Pharaoh.”