The double line is an effct of the sobel filter. The filter works on a pixel, therefor when sobel detects an edge at a pixel, it will detect it too on the other side too:
for understand better the sobel filter:
xxoo xxoo xxooapply sobel on all x
apply soble on all o
There are several solutions:
1. Use the stencil buffer to detect the edge only on parts of the screen, e.g. the x surface only.
2. Use double resolution and scale it down afterwards (or use multisampling).
3. Use an other AA technique to make it less obviously.