Whatever system you come up with, it won't be fair to someone or other. shooting 5 extra targets at the end, means a load of time waiting around and organising people, the person who's just shot in the 2nd session and is fresh from shooting might have an unfair advantage over someone who's sat around eating burgers and drinking sugary tea etc. Some people shoot the 1st session and have to leave early so won't be around for a shootoff at the end etc etc.
I'm afraid, Life just isn't fair.
The best way to avoid an unfair placing because of countback is to knock one more target over in the competition, make it 2 just to be sure