In Lech Lecha, we hear of Sarah as Avraham's sister, in terms of Pharaoh. In Vayera, a similar story, with Sarah as Avraham's sister, in terms of Avimelech. Is this evidence of multiple sources, where there were two variants of the story, and both were recorded? And is the similar story involving Yitzchak and Rivkah another source?
I don't believe so. In terms of Avraham, we see that this is a regular tactic Avraham and Sarah use. In Vayera, in Bereishit 20:13:
So how come we do not hear about all those other places? Because nothing eventful happened in all those places. The people in those places were righteous enough and did not seize Sarah, and did not kill Avraham either.
Why would he do this after the first incident with Pharaoh? Because Pharaoh was an anomaly, perhaps, but in the thousand other places the strategy worked well, and perhaps better than the alternative of revealing it, in Avraham's reasoning. And it is no surprise for Yitzchak to follow in his father's strategy as well, where one incident occurred with no repercussions - just the king realizing that it was a trick.