Hi Asma,
I guess your video source is interlaced. That is why you are seeing two frames in single frame buffer. Though I feel there is some sync issue between source and processor.
There are some video related software modules available on the ADI web-site.
http://www.analog.com/en/processors-dsp/blackfin/content/blackfin_software_modules/fca.html
Hope you may find the Image Processing Toolbox helpful.
-Prashant