How to count person in video?

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I am use opencv 2.4.9 and python 2.7.6, I need to count people in video. What I already did is:

1. Background subtraction
2. Find contour and put the bounding box
3. Find the centroid (cx,cy)
4. Draw a virtual line at (20,75) to (320,75)

I am trying to count people, when they cross the line. I try this code to count it

if 75 <= cy <=77:
   counter=counter+1

no error, but the counter count it double, Is there other way to count it?