can i exhaust a unit already exhaust using nami headstrong to get nami's trigger ?

No — you can't exhaust an already-exhausted unit to satisfy anything, and that isn't what Nami, Headstrong's trigger asks for anyway.

Exhausting a unit that's already exhausted does nothing: "A Game Object that is already Exhausted cannot be Exhausted again," and if something instructs it to be exhausted, nothing additional happens (Rule 414.1.b–c). If the exhaust is a cost, the action must actually be completable to be paid, so an exhausted unit can't cover it (Rule 414.4) — meaning no activated ability, additional cost, or "when you exhaust" condition gets fulfilled off it.

Also worth flagging: Nami, Headstrong has no exhaust ability. Her trigger is "When I hold, the next time you play a unit this turn, ready it and Buff it" — a Hold trigger, which only requires you to be holding the battlefield during your Beginning Phase. Nothing about exhausting a unit gets you there.

If you were thinking of a different Nami or a specific card with an exhaust cost, tell me which one and I'll check it.