Apple iCloud Family Sharing
If I have share the 200GB family plan and I invite someone who has also bought the extra 200GB, will it combine or will it cancel it out?
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The other person will have the option to stay with their own plan or cancel it and share your plan. See the following from Share iCloud+ with your family - Apple Support
If a family member is already paying for their own iCloud+ plan, they can choose to switch to your plan or keep their own and still remain part of the family. When they switch to the shared family plan, they get a refund for the remainder of their personal plan. They can't keep their current plan and use the shared family plan at the same time.