Should I have checked? In retrospect this is the classic KK nightmare. How would you play this? Is this a set line, or another PP defending?

Obviously my preflop bet was too weak. With two limpers it should have been at least $2.50 to go.

PokerStars No-Limit Hold'em, $0.25 BB (6 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver)

UTG ($24.25)
MP ($27.10)
CO ($30)
Button ($24.65)
Hero ($27.50)
BB ($42.20)

Preflop: Hero is SB with K, K.
1 fold, MP calls $0.25, CO calls $0.25, 1 fold, Hero raises to $1.75, BB calls $1.50, MP calls $1.50, CO calls $1.50.

Flop: ($7) 8, 7, 6 (4 players)
Hero: bets $5
BB: folds
MP: raises $7.75 to $12.75
CO: folds
Hero: ??