Why hough Transformation detect Same line twice

After using canny edge detector my image looks like

Then I use Hough transform to extract line. Sometimes I able to find four line. But sometimes Same line detect as two line.

I want to find four corner of the rectangle, so using hough transformation I try to take four line then solve them. But extra line gives me pain. How I can solve this problem.

For hough transformation I'm using modified code of http://vase.essex.ac.uk/software/HoughTransform/HoughTransform.java.html

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