No insurer doing in this State the business specified in subdivision 3301(a)(2) of this title shall make or permit any unfair discrimination between individuals of substantially the same hazard in the amount of premium rates charged for any policy or contract of such insurance or in the benefits payable thereunder. This section shall not prohibit different premium rates, different benefits, or different underwriting procedure for individuals insured under group, family expense, franchise, or blanket plans of insurance.