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.