So I hear you can find mutliple weapon schematics fro some weapons.
Once you build the weapon are the schematics used up?

how would someone go and get those special weapons repaired, since you need a second one to use the repair skill.

Nope, schematics are permanent. Once you've got one, it's yours forever and you can make as many copies of the item as you can find the parts to make.



By making another copy of the item and using that one to repair the first one.

http://fallout.wikia.com/wiki/Fallout_3_crafting

Having more than one schematic gives you a bonus whenever you make that custom weapon. In the case of custom guns the weapon you make will start with a better condition and in the case of mines and grenades you get more explosives from the same amount of ingredients.

Continuing from previous post, so you might want to wait to make three nuka nades at a time, until you have all three schematics.