Leather repair kits don't check whether an piece of leather armor is fully repaired. Crafting a piece of leather armor with a repair kit uses up one kit. If an armor piece is at 100% durability then a kit shouldn't be used up nor should an output recipe show up.

Edit:
If you put in three or less repair kits spread out, they get used up (pants as an example). Any more then they will be returned to the crafting table. It seems that there are different amounts for each armor piece.

If you have any amount of repair kits in one slot along with an armor piece, it will use one repair kit from the stack. So something wonky is happening with the repair recipe.