10th-level Warlocks with a Great Old One patron gain the Thought Shield feature (PHB, p. 110):
You also have resistance to psychic damage, and whenever a creature deals psychic damage to you, that creature takes the same amount of damage that you do.
In this question, it is said that Thought Shield deals psychic damage back to the damager, who benefits from resistance to psychic damage if they have it.
Let’s imagine Alice the Warlock, who has the Thought Shield feature from being a 10th level Warlock with a Great Old One patron, casts synaptic static on Bob, who is also a 10th level Warlock with a Great Old One patron.
Does the damage keep going back and forth between them, and it is cut in half with each reflection? (Mathematically, I believe Bob would take 4/7 of the damage rolled, and Alice 2/7, in that scenario.)