Surjection iff Right Inverse/Non-Uniqueness

Theorem
Let $S$ and $T$ be sets such that $S \ne \varnothing$.

Let $f: S \to T$ be a surjection.

A right inverse of $f$ is in general not unique.

Uniqueness occurs $f$ is a bijection.

Proof
If $f$ is not an injection then:
 * $\exists y \in T: \exists x_1, x_2 \in S: \map f {x_1} = y = \map f {x_2}$

Hence we have more than one choice in $\map {f^{-1} } {\set y}$ for how to map $\map g y$.

That is, $\map g y$ is not unique.

This does not happen $f$ is an injection.

Hence the result.