Board Thread:Questions and Answers/@comment-5149755-20150409001207/@comment-25874178-20160210201652

What Kirito meant when he said "It just appeared", is that the Unique Skill Dual Blades formed part of his List of Skills, under the Skills menu, suddenly. He did not do a quest or get it in a reward, but instead he saw it by checking his Skills.

Extra Skills do need a Skill Slot available to be Equipped, but not to be stored in the Skills Menu.