Double Lines have unique properties that you can set in the Double Line tab of the Properties window.
Separation: The distance between the lines. If you stretch or scale a double line, this distance does not change.
End Caps: Closes the endpoints of the double line.
End Cap at End
End Cap at Start
Tip: Another way to create end caps is to use the Edit Tool (node editing), right-click on the end node and select Closed Double End.