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You are correct. Only the leaking part needs to be replaced. However, maybe the expansion tank is leaking, but they see an imminent leak in the radiator or some other issue with it. If that is the case (and you trust them) then it would be best to replace both at the same time. You don't want to drive off only to have the rad spring a leak next month and cause you to overheat the engine. They might be doing you a favor by pointing out an imminent failure.
My advice would be to ask them further questions about why they are suggesting to replace both, if its really needed right away, ask them to show you the leaks (point them out on the car), etc.
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