The Duplication In Sarah-As-Sister Stories

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:

יג וַיְהִי כַּאֲשֶׁר הִתְעוּ אֹתִי, אֱלֹהִים מִבֵּית אָבִי, וָאֹמַר לָהּ, זֶה חַסְדֵּךְ אֲשֶׁר תַּעֲשִׂי עִמָּדִי: אֶל כָּל-הַמָּקוֹם אֲשֶׁר נָבוֹא שָׁמָּה, אִמְרִי-לִי אָחִי הוּא.

13 And it came to pass, when God caused me to wander from my father's house, that I said unto her: This is thy kindness which thou shalt show unto me; at every place whither we shall come, say of me: He is my brother.'

So we see that they do it in every place.

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.