The question is actually rather straightforward and many people seem to be confused about it:
When a 6th or higher level paladin that has Aura of Protection rolls a Charisma save, does he essentially add his Charisma modifier twice in that kind of roll?
Effectively, yes.
The source of confusion is likely a gut reaction against the idea of stacking the same bonus with itself. However, that's not what's happening here. The paladin is not getting their Charisma modifier twice: they're getting their Charisma modifier once, and also getting a different bonus that just so happens to often (but not always) be the same number as their Charisma modifier:
the creature gains a bonus to the saving throw equal to your Charisma modifier (with a minimum bonus of +1)
(emphasis mine)
Two separate bonuses, one derived from the other, and the game doesn't care about that derivation: they're still different and nothing prevents them from stacking. The wording appears to be designed specifically to make the Aura work in exactly this way.
My first reaction would be to say no... but after reading the text of Aura of Protection, indeed that would be the case: yes, you add your Charisma modifier twice to the die roll on Charisma saving throws. The first time you add it, its in the form of an ability modifier, and the second is a bonus modifier.
What might get people confused is that the rules disallow adding your proficiency bonus more than once to a roll, but in this case, you are not adding proficiency twice, but your Charisma modifier.
