How to Prevent Flare-Ups
Two words: indirect heat. A two zone charcoal configuration is a great way to apply indirect heat - especially when the duck is cooking skin side down.
The grill's top rack isn't just for vegetables anymore. If indirect heat can't cool things down, keep the duck out of the fire's reach.
Indirect heat and top rack placement are just ways to (you guessed it) control the temp. One of those options or a combo of the two should do the trick.
Clean your grill. A cleaner grill means less residual fat and oils that can catch fire. Make it a habit to clean everything up before your next duck cookout.