menconder duplicate frames always

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HOLA FRIENDS OF THE LIST, I HAVE ONE BIG PROBLEM (MY ENGLISH,JEJE) 
WHIT MENCODER, THIS IS MY SCRIPT TO RIP DVD IN ONE PASS :

mencoder -dvd 1 -o /Varios/lapeli.avi -vop scale=512:384 -oac mp3lame 
-lameopts cbr:br=96 -ovc lavc -lavcopts vcodec=mpeg4:vbritate=1400 

THE PROBLEM :
MENCODER DUPLICATE FRAMES ALWAYS ,EVERY SECOND OF THE MOVIE, 

MY PC:
PENTIUM III 1GHZ
360 MB RAM
GEFORCE 2 MX 200
SOUNDBLASTER LIVE VALUE

PLEASE I NEED A SOLUTION 
THANKS FOR YOUR HELP

ADIOS, FROM ARGENTINE

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