I am sorry for wrong information
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The first test’s I did with 720 50p was on JVC mpeg2 stream and used mpegsteramclip – and I could chance frame rates in there.
So what you can do is capture 50p bring that clip into FCP, retime it to 200% and uncheck the frame blending option.
Otherwise FCP brings in the 50p as 25p and you get slow mo effect
-Kaspar