Not sure if it's something you have drawn or auto traced.
If it's drawn you may be getting double lines if you have it filled when you convert to dxf as it makes a line for the line and a line for the fill (even if your in wireframe the fill is there if it's filled, just hidden).
If it's an auto trace maybe you forgot to ungroup from the background after tracing.
I use X5 and I don't use dxf tool so not sure if that adds anything, I wouldn't have thought so.
Reducing nodes shouldn't make any difference but try selecting just one node and dragging it, that will reveal any duplicate lines underneath.
If all else fails there is a macro you can use apparently, I haven't used it but here is a link back to Marty's thread
http://www.plasmaspider.com/viewtopic.php?f=49&t=10248
Let us know how you get on.
Murray