I don't know anything about Sigmanest, but it seems to me it's a setting with Sigmanest.
Maybe a setting to merge overlapping entities?
Are you saving it as a dxf from the model mode or drawing mode? I have never had this issue when exporting from the model mode.
My guess is that the double arc issue comes from the plate not being flat, are you saving from the drawing file? You should be able to save out a perfect part from the part file, by selecting the proper face.
If you're saving out using a macro or saving from a drawing, open up the part file and go to your flat pattern configuration and select the face you want saved, hit the spacebar, go to "Normal to Face" view and save a "New" view, then in the drawing file make sure it's orientated to the new view..
I use sigmanest quite a bit. Rarely have any issue if i export DXF directly from the model view as previous replies explain. One frustrating thing about Sigmanest is it will not recognize poly-lines used to construct an arc, and proceeds to tell you the part has open contours. I would imagine if you are having double lines its due to the fact that you are exporting your dxfs from a flat pattern view in the drawings. The edge of your part may be cut at a slight angle so if you were to look at the "front" face of the flat pattern and the "back" face of the flat pattern they would have very slight differences in arc diameter.