There's a fine line between charred and burned duck breast. Here are some tips for avoiding the latter:
1. Place duck on top rack or over indirect heat
Yes, throwing a duck breast on a burning hot grate is awesome: the sizzle, the smoke, the instant grate lines. Unfortunately, this situation will quickly spiral out of control, turning into a duck-fueled fire ball - cool looking, but not tasty.
There are two ways to avoid this outcome. First, use the top rack of the grill. It should keep the fire under control and keep the duck out of reach of the occasional flame up. Second, if the top rack isn't available, place the duck over indirect heat. Indirect heat is PLENTY hot, especially with a medium-rare finish in mind.
Pro tip: use a combo - top rack + indirect heat
2. Turn down the gas
Sounds obvious, doesn't it? So don't forget it.
Enjoy BBQ Season!