I use GRASS to produce contours from DEM (r.contours for SRTM rasters). They looks fine, but most of the line features occures to be duplicated; i.e. verticies duplicated. Here are pictures of output line (first) and line with some moved vertices (second) to make my problem more clear
This happens to contours, which are not closed. So, they looks fine, but after generalization occure lots of artifacts, and this is unsuitable for relief visualisation.
Could somebody point out the reason of this duplication or offer a way to correct them?