I recently sweated copper pipe, and wouldn't you know it, out
Of approximately eight joints, there was one small drip. What's
The best way to fix this--- do I have to sweat off the piece that's
Leaking. It's leaking at a 3/4 inch union. Can this be reused if I
Can get it apart? Or is this already sweated union now garbage?
Do I have to cut it out and redo it completely. Very frustrated.

Pulling it apart, cleaning, fluxing and completely redoing it is the best approach -- Reheating the joint would actually be just 'tinning', instead of soldering (any flux left in the fitting would have been washed out from the pressure of the water being turned on).

You're lucky, though, it couldn't have happened at an easier joint to repair.

If you previously or in taking it apart over heat the joint it will not solder well.