Parallel path

I don't have time to write this, but i'm sure this will work for any a single open path.

• Deselect all
• Duplicate the path.
• Give it a stroke of say 10 points, No end caps
• Set the path as selected
• Menu Command > Outline Stroke
• Get the newly closed path shape from document.selection
• Given the two end points x1,y1 and x2,y2 from the ORIGIONAL path, test all nodes in the outline stroke path, and find 2 consecutive points which both are within 5.1 points of x1,y1 (10/2 + a fudge factor) and remove this line. The only match should be the two straight lines at the end as only their nodes with be within the red circle below.
• Repeat for multiple outlines
• 

Curved parallel lines.

https://upload.wikimedia.org/wikipedia/commons/thumb/f/f7/Offset-curves-of-sinus-curve.svg/1200px-Offset-curves-of-sinus…

Sorontar​,

please show something - maybe the expected result.

what kind of lines? straight, it might be possible. Curved lines, probably not that easily.