I’m assuming you are working in the Part Design workbench. Select the sketch and click **Edit** > **Duplicate selected object**. Make sure that only the sketch is copied (depending on the version, either deselect the plane or click don’t include dependent objects). This will produce a duplicate sketch in the active body (if you want the duplicate in a different body, make sure it’s active first by double-clicking it if necessary). You can then click **Sketch** > **Reorient Sketch** to move it to a different plane. 

By the way, FreeCAD has a very active and helpful forum at https://freecadweb.org. I highly recommend asking FreeCAD questions there.