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Certainly new filter new flat is best practice in case there are differences but if you keep your filters clean you should be able to get away with one flat for rgb and one for luminance. I have done that at times when I couldn't get or didn't have every filter's flat.
I generally though take flats for each filter. If you use dusk flats you need to increase the exposure time a bit for each one as the light fails.
Greg.
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