The spell Black Tentacles (listed as Evard's Black Tentacles on PHB p. 238) has the following text:
When a creature enters the affected area for the first time on a turn or starts its turn there, the creature must succeed on a Dexterity saving throw or take 3d6 bludgeoning damage and be restrained by the tentacles until the spell ends. A creature that starts its turn in the area and is already restrained by the tentacles takes 3d6 bludgeoning damage.
Each of the two sentence has a condition that results in damage. Note that there is no "or" or "instead" in the second sentence. The second condition is more specific than the first, but it never says they're exclusive.
Does this mean a creature starting its turn restrained by the tentacles receive the damage twice?