Chinese plugin PowerCEMF,one of the main function is to clean up all the crossover lines and full-in the outline in faces after we import Autocad DWG file to Sketchup.
Here is another discussion link I found ( http://forums.sketchucation.com/viewtopic.php?f=323&p=330499 )
Video Link below, (click on any of the dark green button to start download, the link is safe and I already downloaded the video)
下载PowerCEMF视频.swf
Plugin download link from their website(English version), but we can't use it before we buy it online to get the key 
http://www.powersutool.com/downfile/PowerCEMF0.1b2011.1.3.rar
Their website http://www.powersutool.com/?page_id=27
It will be great if anyone can provide some plugins c/w similar function for free 