'Double Strike' was introduced in the Scourge set and it currently appears on a number of cards including Battle Mastery and Marisi's Twinclaws

702.4. Double Strike

• 702.4a. Double strike is a static ability that modifies the rules for the combat damage step. (See rule 510, "Combat Damage Step.")
• 702.4b. If at least one attacking or blocking creature has first strike (see rule 702.7) or double strike as the combat damage step begins, the only creatures that assign combat damage in that step are those with first strike or double strike. After that step, instead of proceeding to the end of combat step, the phase gets a second combat damage step. The only creatures that assign combat damage in that step are the remaining attackers and blockers that didn't have first strike as the first combat damage step began, as well as the remaining attackers and blockers that currently have double strike. After that step, the phase proceeds to the end of combat step.
• 702.4c. Removing double strike from a creature during the first combat damage step will stop it from assigning combat damage in the second combat damage step.
• 702.4d. Giving double strike to a creature with first strike after it has already put first strike combat damage onto the stack in the first combat damage step will allow the creature to assign combat damage in the second combat damage step.
• 702.4e. Multiple instances of double strike on the same creature are redundant.

Translation:

• 702.4a. Double strike is always active and does not trigger or become activated.
• 702.4b. Double strike creatures deal damage in both first strike and the normal damage dealing step.
• 702.4c. If you remove the creature from combat during the first strike damage phase, then it won't deal normal damage.
• 702.4d. If a creature had first strike, had dealt first strike, and then is given double strike it will deal damage during the normal damage dealing step.
• 702.4e. Multiple instances of double strike on the same creature are redundant.