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.