trade iPhone 18 pro for duo?

Apple's not currently taking trade-ins for the iPhone 18 range. Sell it to someone else, or check if you qualify for the store's return policy.

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No. Apple won't take a currently model in trade until the next generation releases, which will likely be next September. Now, if you purchased the iPhone 18 Pro less than 14 days ago, you can return it for refund, then buy the Duo. You could sell your 18 Pro privately, then use whatever you get for it towards the Duo. But trade it in at this time, you cannot do.