"Did not he say "she is my sister", and she herself said "she is my brother"? With an innocent heart and clean hands I did this." (Breishit 20:5)
She herself - The repetition indicates the inclusion of her slaves, camels, and donkeys - "I asked them all and they said she is his sister" (Rashi there)
Such a weird interpretation - the animals tell Avimelech that Sarah is not married? What could force Rashi to say this?
There are several reasons to think in the story that Avimelech's actions might have been morally problematic on some level. A middle-aged man and woman who travel and live together are usually married, not brother and sister. Perhaps Avimelech should have known this and been more hesitant to take Sarah, even after she said she was single. This sense is reinforced by what comes later: Avimelech's household is in fact punished and God does in fact curse Avimelech very harshly.
I think Rashi wants to emphasize that, despite these considerations, in the final analysis Avimelech did nothing wrong. Rashi does this by changing the original circumstances to make Avimelech's action more justifiable. Asking the slaves, of course, simply provides more voices testifying to the same point, and making that point more believable. Asking the camels and donkeys has a qualitatively different significance. It indicates that just as Avimelech seemed to receive punishment and condemnation through miraculous means, his original actions were also justified by a miracle - by the animals telling him that Sarah was unmarried. Thus we have justification against punishment, miracle against miracle, and there is no longer a reason to say that Avimelech was guilty.
Then Abimelech called Abraham, and said to him: "What have you done to us? and how have I sinned against you, that you have brought on me and on my kingdom a great sin? You have done deeds to me that ought not to be done."
And Abimelech said to Abraham: "What did you see that made you do this thing?"
And Abraham said: "Because I thought: Surely the fear of God is not in this place; and they will kill me for my wife's sake." (Breishit 20:9-11)
Why does Avimelech speak twice? He is saying very similar things, apparently part of the same speech. But the phrase "And Avimelech said" is repeated twice. Why the repetition?
I think the text is trying to emphasize that the two halves of Avimelech's speech were, effectively, separate from each other. The first half can be summarized as "How could you have done this to me?" It is an expression of indignation, not a question. Avraham cannot answer it, but he does hang his head silently and implicitly concede that he is guilty. The second half can be summarized as "What motivated you to do this?" Once Avraham has admitted that he is in the wrong, Avimelech asks another question, more out of incredulity than anger. What could possibly lead Avraham to have done what he did? For this Avraham can, and does, supply an answer.
UPDATE: In a shiur from R' Yehudah Trofer I was introduced to the definitive article "Vayomer Vayomer" by Meir Shiloach, which discusses about 100 cases in Tanach where "he said" or similar language is repeated within a speech, and divides the cases into seven distinct categories. This would seem to be one of the cases in his first category - when a response was expected but did not come. Nechama Leibowitz is also said to have an article on the topic.
"And it came to pass, when God caused me [Avraham] to wander from my father's house, that I said to her: This is thy kindness which you shall show me; at every place we come to, say of me: He is my brother." (Breishit 20:13)
The repetition of the "she is my sister" stories in Breishit - twice with Avraham, once with Yitzchak - is strange. But this verse indicates that Avraham (and presumably Yitzchak) felt the need to tell the same lie wherever they went. Thus, it is understandable that the erroneous marriages repeat themselves. It is less understandable why Avraham and Yitzchak would tell such lies, abandoning their wives to other men, in the first place.
R' Yoni Grossman suggests that we explain their motives by looking at ancient (and modern...) Middle Eastern society. In Biblical times, a woman could not just go marry whomever pleased her. Rather, the suitor would have to approach her father or closest male relative. The suitor and the male relative would bargain. Eventually they would reach an agreement by which the woman was married off and her family got a sum of money.
Whenever Avraham said that he was Sarah's sister, people thought of him as Sarah's closest male relative. People who wanted to marry her had to ask Avraham for her hand in marriage. But Avraham could always reject them by asking for more money than they could afford. And Avraham could play the suitors against each other. "You're offering 50 sheep for my sister, but I just received an offer of 60 sheep for her. So why should I give her to you?" While Sarah was nominally on the marriage market, Avraham could ensure that she was not actually married off. Whereas if Sarah had admitted to being his wife, there would have been no legal possibility of marriage, and one of them might have killed Avraham and taken Sarah by force.
It was a dangerous game. But it in fact worked perfectly in Canaan and for a while in Egypt and Gerar. There was just one flaw. Bargaining worked fine with ordinary citizens. But if the king became interested in Sarah, then Avraham's strategy was suddenly worthless. The king could not be out-bargained, and if Avraham simply refused to marry her off, the king had the power to take her by force. Thus, despite Avraham's planning, both Pharoah and Avimelech were able to "marry" Sarah against her will.
At this point God had to interfere to rescue Sarah. Avraham's strategy was clearly successful and justified in regards to the ordinary citizens of the land. The question is whether Avraham should have foreseen that the kings would notice Sarah. I think the answer here is: in the first case (Pharoah) no, in the second case (Avimelech) yes. In Egypt it is clear that Pharoah did not see Sarah himself, but rather Pharoah's servants saw her and reported her to Pharoah, leading to Pharoah taking her. This seems to have been totally unexpected, and Avraham is not held to blame for it in the story. But with Avimelech things are different. Having run into a lustful king once, Avraham perhaps should have expected it to happen again. Thus, in the Avimelech story, it is Avraham who is to blame. According to some interpreters, the Akedah command (which follows two chapters later) is a punishment for Avraham's actions with Avimelech, and through its successful conclusion Avraham reaffirms the covenant with God which was damaged by his behavior in regard to Avimelech.