How to mutate a Vec<u16> in a struct in a DoubleStorageMap?

Question

How do I mutate a DoubleStorageMap with a Vec<T::AccountId> as a value using a struct?

Data::<T>::mutate(
  id.clone(),
  user_id.clone(),
  |params: &mut DataParams<T::AccountId>| {
    params.users.push(new_user);
    params.total_users += 1;
  }
);

total_users doesn't increase but users only ever has one user.

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What's the best way to accomplish this? The mutate is within a for loop and needs to append multiple users.