Double stitch 
is a row of small stitches going forward, followed by a row of small stitches going backward, so it ends at the same point where it began.
Use double stitch for markout and to place branching outlines that need to return to their starting points.
To show the Double Stitch dialog
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Click a Double Stitch line so that the points defining it are shown, then Control–click and choose Properties.
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Select Double Stitch in the line pop-up menu in the stitch type section of the Create toolbar, then click the Options button and select Fill Area and Line from the pop-up menu. In the Area Properties dialog, click Line.
Length
Set the Length of Double stitch from 1 to 12mm in steps of 0.1mm.
The initial setting is 2mm.