It is Not Duncan; as viper_390 mentioned, it is Turbinaria. Much more common that Duncan (in the US at any rate). Slower growing, and does not eat as much as a Duncan will (though it will still feed if given a chance).
Not likely to branch out, they typically encrust, but will sometimes develop folds or 'fingers' in the middle of the plate.
As far as the confusion goes, they are actually closely related to Duncanopsammia (and Tubastrea, etc).