Or, when you are sketching, choose the existing line as a reference and just start sketching parallel lines. You might have to sketch centerlines perpendicular to your reference line at the reference line ends. This way might be the quickest way to duplicate lines. Once you have sketched so many lines, you can then use the selection tool to window your lines, right click, choose copy and place your copied lines.
Another neat trick is to sketch lines perpendicular to your copy line, make them construction lines with a constraint of equal to the first sketched perpendicular construction line. This way you have one dimension, not many controlling your sketched offset lines.