It only reset the amount on the ground, so that foes will continue to take damage. It won't increase.
Be carefull, if you used any special caltrops, with nice poison on it, and then throw classic one, you will remove the poison one. Best to always put poison caltrops last.
A good way to somewhat increase caltrops damage, is to combine with acid with cooked shot acid gun, or with flame thanks to incendary grenades or other flames means.