Standard recruiting keeps duplicating. Do you discharge all lower recruits?

The original question was about troops not heroes. Yes you upgrade the lower troops by clicking on them when starting training and then bottom middle of screen click on the amount you have then click train and it will upgrade the selected troops.

I used the duplicates to upgrade all the C and B class heros to 6 Stars, then discharge them to get credits to level up the skills of the S and A classes. Also discharge the soldier credits you get when you get a new hero.

When B and C level heros are maxed then you can discharge them the point you get from dischargeing you can use them to upgrade hero skills