To copy a stroke you've drawn, Select the stroke, then tap the Duplicate icon in the Selection popup above the selection. The duplicate will appear Selected, ready for you to edit or move on the canvas. Tap to set it in place.

 

Alternatively, you can copy a Selection to the clipboard to move between drawings. To add a stroke to the clipboard, Select the stroke, then tap the Paperclip icon in the Selection popup above the selection. To paste the Selection, tap the down arrow icon in the status bar and choose paste from clipboard.