The duality involution is self-adjoint. More generally, this is true of the underlying category of any 2-category with a duality involution.
If a category has biproducts, then the composite of the (discrete -ary) diagonal functor with the (-ary) biproduct functor is self-adjoint.
Functors self-adjoint on the left
There is a similar phenomenon involving a change of variance. A functor is called self-adjoint on the left if . In this case, we have a natural isomorphism . (Conversely, we may talk about functors self-adjoint on the right if .)
The contravariant powerset functor is left-adjoint to , i.e. self-adjoint on the right.
More generally, in a symmetric monoidal closed category , for a fixed object , the functor is self-adjoint on the right.