Lol, so i should even give the users that got paid 2.5 days it back. Come on, srsly ? Remember when btcguild paid out 1250BTC too much ...
And no it's not that easy to tell who got paid double, there is no diffrence in the db if a user is using stratum or getwork.
Yes you should pay back _everyone_ you took money from. Like you said you can't tell who did benefit and who didn't. It was your mistake you should pay for it.
You sir, are living in a dream world.