Uniqueness (up to natural isomorphism) of equivalences

Question

Let $C$ and $D$ be two categories and let $F:C \to D$ and $G:D \to C$ be two functors forming an equivalence of categories. Given another functor $G':D \to C$ that together with $F$ forms an equivalence, is there necessarily a natural isomorphism between $G$ and $G'$?