3DSurvey does a great job creating contour lines but I've found that, when exported as dxf and imported into AutoCAD, contours that appear to be closed are sometimes not closed polygons and can contain overlapping line segments where the polylines would close.  Is there a way to prevent the creation of overlapping line segments?  I usually process the contours in AutoCAD to remove vertices and simplify the contour lines so that AutoCAD can handle them more easily.  Thanks.

I am assuming, you created  Contour lines from the Full 3d mesh. Try creating them from Regular grid mesh (DSM), and you should not have issues.

Contours were created using the regular grid mesh which is what I always use with same results.  However, I've found a work-around in AutoCAD using the "Overkill" command to remove those extra overlapping end segments of the contour lines.