In this scenario the precedent has already been set in the 5th, why would the Supreme Court hear it again?

The 9th has nothing to do with an immigration hearing in the 5th, the individual’s personal history is irrelevant here.

Matter of Rahman, 20 I&N Dec. 480 (BIA 1992) seems to cover this. (IANAL)

The parties involved can always petition the Supreme Court. They're free to take up any case even if there isn't a split between lower courts.

They're also free not to take up the case, and leave the circuit split unresolved. If you want to leave the loophole open, that's a win.

Because it conflicts with the holding in another circuit, called a "circuit split," which is one of the most common causes of the Supreme Court choosing to hear a case. The Supreme Court doesn't want forum shopping.

> In this scenario the precedent has already been set in the 5th, why would the Supreme Court hear it again?

A circuit split on an issue is one of the factors that traditionally weighs in favor of the Supreme Court hearing it, in the interests of uniformity of the law.

If the Supreme Court likes the 5th Circuit rule, they take the case it make it the rule, period.