The SVG export in Rhino 6 is a godsend. But I just now figured out why I keep getting duplicate geometry in my SVGs - the Grasshopper preview is being included in the export! I would expect only selected geometry from Rhino to show up in the file. Exporting to svg with only the rectangle selected in rhino, I get this, including the plane grid and circle from GH preview:
I just painfully banged into this bug too! I export baked GH-created geometry frequently, and for all other formats I’ve used (like dwg, dxf), Rhino appropriately exports only my baked geometry. But I can confirm, as @andheum reported over 5 years ago, that in the case of SVG, it includes GH-previewed geometry as well. Because the preview is hidden under the baked, the duplication is often invisible (except when you are using the file for CNC fabrication 
). A fix would be much appreciated.