When you sign up for the family program, Emirates assigns a new number to each family member you name.
If your family members want to accrue miles for a flight, they would provide the numbers for their individual accounts. If they want to credit you through the family plan, they should use the new number associated with the family plan. You don't have to be flying with them, but you only earn 20% of the mileage they would have earned on their own.
If you think about the purpose of loyalty programs, it would make no sense for Emirates to award full mileage to someone and another 20% to a family member who is not even flying, but regardless, it's also explicitly stated in the Skywards Terms and Conditions:
- A Family Bonus Member, after his/her Family Bonus programme enrolment, earns 20% of the Skywards Miles a normal (i.e. Blue) Member would earn on Qualifying Flights and these Skywards Miles are added to the Family Head's Account. A Family Bonus Member earns no Skywards Miles or Tier Miles in their own right when travelling on a Family Bonus membership number.
- A Family Bonus Member may also be a Member but cannot earn Miles in each membership capacity for the same Qualifying Flight(s).