I have been struggling to eliminate backlash and think I hit on the solution. 

I have tried two brass nuts but that was really difficult to get the tension right. A quarter turn is the difference between to loose and binding up.

Backlash nuts feel better at first but they are easily overcome under load. Plus they make a lot more noise and they add a lot of drag for the motors to overcome.

I recently tried delrin nuts (well imitation ones from amazon). They have much tighter tolerances out of the box and you can more easily double them up. It is still easy to bind it up with two so consider shimming the nut if you do this.

I’m not sure how long they will last but so far the are great and the price is right.