Doublestrike

As of Update 5, a new mechanic has been introduced called "Doublestrike." A character with a Doublestrike chance has a chance to make an additional attack roll with their main hand weapon any time they make a main hand melee attack, on the target of the first attack. Many speed bonuses have been changed to Doublestrike effects. Doublestrike only applies to melee attacks.


 * Fighter Weapon Alacrity capstone feat includes a +10% Competence bonus to Doublestrike.
 * Paladin spell Zeal provides a +10% Sacred bonus to Doublestrike to the caster.
 * Monk Wind Stances grant a +2.5%/+5%/+7.5%/+10% Insight bonus to Doublestrike chance instead of an insight bonus to attack speed. (The enhancement bonus to attack speed remains.)
 * Ranger Tempest III enhancement now grants a +5% (unknown type) Doublestrike chance while wielding two weapons. (The Shield Bonus and to-hit penalty reductions that Tempest grants when wielding two weapons remain unchanged.)
 * Bard Warchanter II Inspire Recklessness song gives a +5% Morale bonus to Doublestrike chance and -10% fortification to all nearby allies.
 * Opportunist, a Rogue Special Ability feat, provides a +3% chance to Doublestrike
 * Shield Mastery and Improved Shield Mastery feats grant a +3% / +8% bonus to doublestrike while wearing a shield.
 * Natural Fighting feat grants a +6% bonus to doublestrike.
 * Some weapons include a bonus to Doublestrike. It isn't known if this bonus stacks with above other sources. It does apply to the main-hand melee attack when the Doublestrike item is held in the off-hand, such as with Seeker, Backstabbing, and the DC bonus portion of Stunning.
 * Timeblade has a +3% bonus to Doublestrike.
 * Hellstroke Great Axe has a +3% bonus to Doublestrike.
 * Epic Timeblade has a +6% bonus to Doublestrike.
 * Epic Hellstroke Great Axe has a +6% bonus to Doublestrike.
 * Cutthroat's Smallblade from the Treasure of Crystal Cove event:
 * Cutthroat's Smallblade (Level 8) (Tier 1) to (Tier 3) has a +3% bonus to Doublestrike.
 * Cutthroat's Smallblade (Level 12) (Tier 1) to (Tier 2) has a +3% bonus to Doublestrike.
 * Cutthroat's Smallblade (Level 12) (Tier 3) has a +6% bonus to Doublestrike.
 * Cutthroat's Smallblade (Level 16) (Tier 1) to (Tier 3) has a +3% bonus to Doublestrike.
 * Epic Cutthroat's Smallblade (Tier 1) to (Tier 2) has a +3% bonus to Doublestrike.
 * Epic Cutthroat's Smallblade (Tier 3) has a +6% bonus to Doublestrike.
 * Swashbuckler from the Treasure of Crystal Cove event:
 * Swashbuckler (Level 4)(Tier 1) to (Tier 3) has a +3% bonus to Doublestrike.
 * Swashbuckler (Level 8)(Tier 1) to (Tier 3) has a +3% bonus to Doublestrike.
 * Swashbuckler (Level 12)(Tier 1) to (Tier 2) has a +3% bonus to Doublestrike.
 * Swashbuckler (Level 12)(Tier 3) has a +6% bonus to Doublestrike.
 * Swashbuckler (Level 16)(Tier 1) to (Tier 3) has a +6% bonus to Doublestrike.
 * Epic Swashbuckler (Tier 1) to (Tier 3) has a +6% bonus to Doublestrike.
 * Alchemical Crafting Tier 2 Martial Air effect includes a +6% bonus to Doublestrike.
 * Cannith Combat Infusion on Fabricator's Bracers grants a 5% alchemical bonus to Doublestrike.

More info can be found here http://forums.ddo.com/showthread.php?t=338851