I have 3 small colonies of duncans in my tank. They all seem to be open the most during daylight light settings. I've noticed mine are my first indicator of issues in the tank (rics being too close, doser not functioning properly) and they don't seem to like direct flow. One was a 3 polyp frag I brought home from an all day road trip six months ago and now has close to 20 heads.
I have 3 small colonies of duncans in my tank. They all seem to be open the most during daylight light settings. .
We've had a duncan for about a month or so and it has about 4 new heads already. I feed the corals and fish daily so maybe that is why. But I've also been told they are really fast growers in general.
Ours is always out when the lights are on and closed up when they are off.
To be clear, too many feedings can be bad in some corals. Dendros and the likes need to eat and poop it out before being fed again. The food can rot in the skeleton if too much is given too frequently.