Second, NO attack in the game adds half level to damage.
Third, the feat you're talking about (admixture breath) is a paragon feat.
Fourth, that feat doesn't increase damage, it merely changes the element.
Fifth, you can only take a feat once unless it explicitly says so in the feat description; this one doesn't.
Sixth, game elements never stack with themselves, so even if it gave a damage bonus it would not do so twice.
I'll leave it as an excercise to the reader as to whether this guy is mistaken on how the rules work, or is a cheating munchkin.