One way to avoid double lines is to use the paint bucket tool instead of autotrace. If you click on a color on the outside of the line it outlines the outside of the outline and if you click on the inside of the shape it outlines the inside of the outline. There is even a setting that lets you tell it to ignore small gaps and to let you give the outline an automatic outset. Plus, it is really fun to play with!
I found this by accident, and then read about it here
http://tavmjong.free.fr/INKSCAPE/MANUAL ... ucket.html
Give it a try...you will be amazed.