You could detect them as follows:
- run Canny --> you'll get two lines for the thick ones
- then HoughLines or HoughLinesP
- if you use HoughLines: you get a lines array represented by rho and theta. Thus, you can group by theta, then you get all lines which have the same angle, afterwards you group those togeter which differ from their rho-Value only by a small amount
- if you use HoughLinesP you would have to compute the slope yourself from the line-segment-ending points, then group those with a similar slope together and afterwards check how far the parallel lines are away from each other
