Need Help Duplicate Lines

The Unite button in the Pathfinder panel is what you're looking for. It'll merge all the selected paths together into one, which effectively eliminates double lines, which are usually caused by two paths butting up against each other.

 

this is probably happening because many of those inner shapes are not closed. They are just a stroke line with no fill. The only way to fix this is to convert all strokes to shapes. It is called object>path>outline stroke. You will also have to get rid of any fills otherwise the unite function will end up combining both the fills and the converted strokes into one. Even after doing this it may not work. You have to work on one shape at a time getting rid of strokes, overlaps, hidden shapes, etc.

I do work like this on a daily basis and the foolproof way takes too much explanation to type out. This motorcycle could easily require up to an hour of work fixing all the issues and getting the result you want. The unite function won't work on open overlapping shapes

 

If you use control-Y to show outlines and it looks like a pile of random squares all over the screen I wouldn't dig too much. I deal with a lot of files from overseas that aren't actually created in Illustrator but they save it out as an AI..and spent hours fixing them so I can actually navigate the files. I would just lock the Ai down and trace it for the cut. It seems like a lot of work, but nothing compared to digging through and getting all the gradients and colors to not be masked squares.

 

The file itself will need to be gone through spending an hour or so with selecting the individual elements and uniting / welding them together with one another to get what you're trying to achieve. For a fast and easy fix, just rasterize the file at a decent quality size then just auto trace the raster image. This will work great for an easy fix to a file that doesn't have a whole lot of blends or gradients. But you might lose a bit of the quality.

 

I would put it on its own layer and click the box next to the "eyeball" in the layers panel. Should turn on a little "lock" symbol. Trace over top in a separate layer.