I thought there was a stealth fix of dual wield, but OH -33.3% Damage only appears to be working upto L10 dark knight. Still -33% from both on the level 57 character. The DK was made a few weeks ago, so believe something changed in the base formulas, but it's still broken at high level.
Normal Attack tooltip says 50% weapon damage.
Actual damage looks affected by enemy armor (damage reduction), but is obviously not 50%.
raw character sheet damage:
dual wield: 54
2h: 49 => 65% raw
dual wield: 36~41 => 67%~76% raw
My L57 has +8% damage bonus from skills and +8.2% general weapons master bonus from item stats.
2h L54 => 113.3% Damage
1h L56 => 116.2% Damage <- looks about right
2h L57 => 112.5% Damage
Gear resistance does not appear to be a factor.