This is a matter of style, and therefore a matter of opinion and prescription. My rule is to write the "s" only when you would say it in speech. That gives us: Euripides' Thespis's Sophocles' Aristotle's
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enoonEuripides' Thespis'sThanks Enoon and CJ. I'm not quite sure if I get the idea here. How come we've got 's added to Thespis but there's no 's in case of Euripedes or Sophocles ?