A question about a semigroup with two elements

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Let $(\{p,q\},∗)$ be a semigroup, where $p∗p=q$. I want to show that:

$$q∗q=q$$

I know semigroups are closed and associative, but I am not able to prove this equality.

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Hint:

Suppose to the contrary that $q*q=p$.

Case 1: $p*q=p$. Try to derive a contradiction thinking about $p*p*q$.

Case 2: $p*q=q$. Same advice as case 1.