Im using XP pro dual core AMD with 3GB
i tried exporting the file to different MOV files and got the same duplicating frames
i also tried to export to AVI and Img Seq and it worked ok so maybe its something with Quicktime
another option i can think is different is in AE 7.0 you can Hold alt and open the Quicktime and i couldnt do that in CS3
does that help???