Try using a vector file format (SVG or DXF) and see if that helps. Currently what you have is not an actual vector image—PNG is a raster format. Note that Luban has bugs surrounding vector image import that you will have to tolerate or work around.
Or you can try alternative software (Lightburn for laser, Fusion 360 for CNC) to generate your gcode, and avoid Luban’s bizarre and buggy behaviour that way.
Try saving as “Plain SVG” format rather than Inkscape SVG format, or try DXF in inkscape, and make sure to set the Viewport size and Document size correctly. Also worth looking at the Line thickness/width on that path in inkscape.
I also had the “double lines” problem when I created shapes with Inkscape. I searched for a long time and found no solution. In https://ncviewer.com I could at least see the double line in the G-code. But I didn’t have the knowledge and the desire to get to grips with G-Code.
Lightburn was my saviour. Just load your double-lines.svg created with Incscape into Lightburn. Scroll into it and you will see double lines. Ungrouped in Lightburn, one line deleted and chapeau!
If you haven’t discovered @Skreelink 's guide yet, check this out.