Lately this question comes up more often, when we talk to prospective clients. It is a result of bad business behavior in some areas.
Banks are known to charge both sides of deals that they broker. Old media companies are charging advertisers for the attention of their readers or viewers, while at the same time charging those same readers for subscriptions.
Whether we are representing a brand owner in licensing a brand or whether we are consulting a licensee looking for a new brand: We only collect from one side, from our client. Because we work and negotiate on his behalf.
Everything else we see as unethical.