PHOENIXZERO
Member
1) S1's entire plot. Fairy tail town where no one remembers life before the town (save for a select few) and everyone is stuck in a time loop (save for a select few).
2)S2: If you leave the town, you can come back, but you forget who you were (assuming you were originally from the Forest and never left Storybrooke and had no magical protection).
3) The second Dark Curse cast removed the memories of the brief time spent between Pan's curse and the casting of the second Dark Curse.
4) This current season.
Personally, I'd really only say this is the second time they've done this, since I don't count S2's plot device (since it only happened to a couple characters) and S1 (since it's the original premise of the show).
It wasn't everyone but Emma and Henry at mid-season last year with new memories in their place.