eww, that hurts  :-)  I once wrote autocad code when I worked for McDonnell-douglas automation in a prior life!  that was decades ago.  I will continue to try to figure out what I am doing to generate extra lines.  it is interesting, when I select lines with a crossing box, sometimes i'll see additional handles, indicating extra lines, but found one case where there were several layers of full lines and partials stacked up and only the endpoint/midpoint handles showed.[just like when there is only a single line]

what I am typically doing is quite simple, stretching the sizes of mortise and tenons on some plywood cnc furniture.  it seems the shrinking adds lines(?).  you're right thought, as IBM manuals used to say "probable user error".