David
i.e you can work with their area,I maen
if rectangle(1+2)<<rectangle1+rectangle2 thath means there is redundant rectangles
rectangle(1+2) is the shape that forms because of redundant rectangles
now I dont know how you could implement it,I think you can work with colors
also there could be other ways but this I told could be the simplest one.
Hth.

--------------------------------------------
Mehdi.:)