"It is what it is" isn't a tautology. It's a condensed way of saying that "it is what is seems to be, and nothing more or less; in particular, it is not something else we might wish it to be."
Well none of the other example repetition usages are simple tautologies either. The entire post is about how repetitions that appear to be simple tautologies are actually communicating some meaning.
If we remove the nuanced semantics, a logical tautology can be discerned in the syntax "it is what it is", namely:
(it = what?) ∧ (it = what?)
The left expression says that "it is what?": it binds the meta-variable what? to the it object. The right expression has the binding of what? in scope already, and just redundantly asserts it; the two what?s unify. The right side is saying "it is what; and as for what that is, see the left side".
It's basically a more elaborate form of the P ∧ P tautology.
BTW: I just intuited that I can get ∧ using Japanese IME by typing "ando" (あんど); it appears as one of the replacements.
Japanese IME is useful not just for Japanese. Need ohms? Just "omega" (おめが) and there it is.