Except ret doesn't beat dk in duel. Not unholy that is.
I'd say it's like unholy dk = feral > ret > frost dk
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Duels, wpvp and random 1v1 encounters in bgs are what makes pvp fun. To dispatch a strong opponent in a pure 1v1 test of skill is the best thing about pvp, not facerolling in an arena or rbg with an army of healers and supporters on you, that's not skill nor is it satisfying.