I agree that there should be a Copy function under the Sketch menu, or at least a Copy checkbox in the Move Sketch dialog (similar to other Move functions). But there is a roundabout way to create a copy of a sketch.
- Create a new sketch
- Click the Move Elements icon (looks like a diagonal arrow and two blue corners).
- In the Move Elements dialog check the Copy box and set Change Owner to the new Sketch.
- Copy the elements from the original sketch into the new sketch.
Not real intuitive, but it gets the job done. This process can also be used to copy or move elements to different views or sheets.