Why did Pharaoh then ask him again why he claimed that Sarah was his sister?
Seforno: He asked him again, because, even after he had taken her to the palace, Avraham continued to maintain that she was his sister, thereby almost causing him to sin.
Under similar circumstances, Avimelech will later invite Avraham to remain in his land (20:15). Why did Pharaoh not do the same?
Rashi: Pharaoh did not invite Avraham to remain, like Avimelech would later do, because the Egyptians were steeped in immorality, as the Navi testifies (Yechezkel 23:20). It would not have been safe for Avraham and Sarah to remain there.
Why did Pharaoh add the word "as a wife"? Why did it not suffice to say that he took her for himself?
Ramban and Seforno: He meant that he took her as queen, and not just as a concubine. 1
Thinking that Avraham would certainly agree.
QUESTIONS ON RASHI
Rashi writes: "'Take [her] and go!' - Unlike Avimelech [who invited them to stay]... Pharaoh said to him, 'Go and do not tarry, for the Egyptians are steeped in immorality....'" What is Rashi explaining?