How much damage does a creature take for spending its entire turn inside a Wall of Fire?

Question

The conditions for when wall of fire deals damage, after the initial casting, are as follows (emphasis mine):

One side of the wall, selected by you when you cast this spell, deals 5d8 fire damage to each creature that ends its turn within 10 feet of that side or inside the wall. A creature takes the same damage when it enters the wall for the first time on a turn or ends its turn there. The other side of the wall deals no damage.

This is a bit confusing, because it seems like there might be two separate effects that can each deal damage to a creature that ends its turn inside the wall itself. Specifically, if the creature starts its turn inside the wall and remains in the wall for its entire turn, it looks like both effects would trigger at the end of its turn, dealing a total of 10d8 fire damage. (It also looks like a creature that enters the wall on its turn and remains there might take 10d8 total, but not all at the same time.)

Is this intended to deal double damage to a creature that spends its entire turn in the wall, or is the wording simply a bit redundant? How much damage does a creature take if it spends its entire turn inside the wall of fire?

Answer 1

It takes 5d8 damage total.

On its turn in initiative, a creature starting its turn in the wall does not take damage. Doing actions in it does not cause the creature to take damage. Ending its turn in the wall deals 5d8 damage.

Therefore, 5d8 damage is dealt to a creature that spends its entire turn in the Wall of Fire.

The sentence structure of the spell - two separate sentences saying that a creature ending its turn in the wall takes the same 5d8 damage - is redundant. Otherwisem the spell would be re-written as, "A creature entering the wall takes 5d8 damage. A creature ending its turn in the wall takes 10d8 damage."

Answer 2

There's a few spells with wording like this; Moonbeam has some janky stuff like that as well. If you logic it out, it's actually specifically designed to prevent a creature from taking damage from that source more than once in a round- in Moonbeam's case because that spell lets you move the beam over an enemy, in both spells' cases because someone might throw a creature into the effect while it's not the creature's turn.

The "On a creature that Ends it's Turn" bit really ought to come after the "...enters the wall for the first time on a turn..." bit, because it's that second bit that gives the relevant information. It's the "OR" that's important here, because without that, you'd get exactly the situation you describe. But with the Or, it becomes:

if (creatureInRangeEndofTurn=True)||(creatureInRangeMoved=True)
     Damage=5d6
;
return 0;

Instead of:

if (creatureInRangeEndofTurn=True)
     Damage=5d6
;

if (creatureInRangeMoved=True)
     Damage=5d6
;
return 0;

The first bit is really the redundant part.