Can we prove a set of a set by the Pairing Axiom?

Question

According to the Pairing Axiom, suppose we have two sets $a$ and $b$, we have a set $\{a, b\}$.
But can we have a set $\{k\}$ from a set $k$ by the Pairing Axiom?

Answer 1

Yes. Just take $a=b=k$. In the pairing axiom, $a,b$ are not required to be different.

Also have in mind that for $a=b$, $\{a,b\}=\{a,a\}=\{a\}$.