Double Lines have unique properties that you can set in the Double Line tab of the Properties window. 
Sepa ration: The distance between the lines. If you stretch or scale a double line, this distance does not change.Ref . *En d 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 *Cl .
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