Any WW2 RAF experts?

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I wanted to know if any of you you guys could tell me. In a unit of Spitfires did all the aircraft have to be the exact same type or would there be a mix of aircraft. Say one unit with MK1's and MK2's and then another unit with MK5's, MK6's, and MK8's and then another unit with a mix of Griffon powered variants. I can find the answer anywhere. I know with AFV's there was definitely a mix and still is in some units.