I'm just a hobbyist, but since no one else has answered, I'll do my best to at least point you in the right direction . . .

The duplicate stories you are referring to are called 'doublets' and are best understood as stories that circulated independently before being collected into the biblical texts as we know them today. The Documentary Hypothesis posits that the first five books of the bible are a blending of at least four original documents/traditions (JEDP), which account for repeated stories such as unique accounts of creation in Genesis 1 & 2 and the intermingled accounts of Noah's flood.

While there are continually squabbles over the dating and assigning of various passages to the different sources, this chart from Wikipedia gives you an overview of the DH:

the Yahwist source (J) : written c. 950 BCE in the southern Kingdom of Judah.

the Elohist source (E) : written c. 850 BCE in the northern Kingdom of Israel.

the Deuteronomist (D) : written c. 600 BCE in Jerusalem during a period of religious reform.

the Priestly source (P) : written c. 500 BCE by Kohanim (Jewish priests) in exile in Babylon.

I don't know that I've ever heard that Abraham's encounter with the Pharaoh was anything more than an etiological tale to explain his wealth. Richard Friedman has this to say about it in his book, "The Bible with Sources Revealed":

There are three stories in Genesis in which a patriarch's wife is represented to be his sister, a king learns that she is actually teh patriarch's wife, and then the husband and wife leave and prosper. The first and third are both J. They do not overlap characters: the first story is about Abraham-Sarah-Pharaoh; the second is about Isaac-Rebekah-Abimelek. The second story combines characters from each of the other two: Abraham-Sarah-Abimelek.

The book also notes that the second story in Genesis 20 is the first occurrence of the source E.

This is a bible study page. I'm also looking at my copy and it seems like the general consensus is that if the wives were pretty, the men would be killed in order to then 'have' the wives.

One point I feel like I should make is that looking at this from a literary perspective, the narrative stresses that this was not ever a real danger-in each story, the claim is made, the woman taken in, and after divine intervention makes clear the marital status of the woman, the king pointedly asks why he was not told she was married and stresses that had he known he would have not done anything.

If you study the relationships you may discover that the wife is actually a half sister. The relationship is true but is not the only relationship.

This allows the implicit boast. The wife is so beautiful that in the land of multiple wives the monarch would kill the husband to add the wife to his harem. Emphasizing the brother sister aspect explains why two adults of opposite sex are traveling together but removes the possibility that a widow can be produced who needs protection by marriage to the monarch.

Sending a servant on a journey to find a first cousin to marry is also a theme. Things like this such as the very early marriage age for women is not emphasized among Christians of our times.