When the wall is created you get can make a dex save for half damage
When cast there is the following effect:
When the wall appears, each creature within its area must make a Dexterity saving throw. On a failed save, a creature takes 5d8 fire damage, or half as much damage on a successful save.
Thematically you can jump out of the way while the fire wall is still forming.
After that, entering the fire will burn you
Thereafter there is this effect:
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.
In the later there is no mention of a dexterity saving throw, or half damage. The fire wall has sprung into being, you cannot "dodge" your way through a wall of fire, if you enter it you will be burnt.
When the second last sentence says "the same damage" it is referring to the previous sentence which says "deals 5d8 fire damage".
In summary
- When the wall is created make a dex save, if you fail take 5d8 damage, if you succeed take half that
- You can move through the wall, but the first time you do so on a turn you will take 5d8 damage.
- If you end your turn in the wall or within 10 feet of the selected side take 5d8 damage.
