A.13.1- The ability is written "Double Strike".
A.13.2- This ability means that the creature deals damage during both the first strike and non-first strike damage dealing steps in combat. [Legions FAQ 2003/01/23] See Rule C.5.9.
A.13.3- This is a static ability (see Rule A.3) that modifies the rules for the combat damage step. [Legions FAQ 2003/01/23]
A.13.4- This is a non-cumulative ability. Having Double Strike more than once gives no additional effect. See Rule A.1.2. [Legions FAQ 2003/01/23]
A.13.Ruling.1- Double Strike is not First Strike. Effects that make a creature lose First Strike will not make it lose Double Strike. [Legions FAQ 2003/01/23]