"If your iPhone keeps reverting back to your old Gmail account when trying to set up a new one, it could be due to cached login information, incorrect account removal, or issues with your Apple ID settings; to fix this, try completely signing out of your old Gmail account on your iPhone, double-check that you're not accidentally selecting it during the new account setup process, and ensure you're not using the same Apple ID email address as your old Gmail account; if the problem persists, you may need to check your Keychain settings or contact Apple Support for further assistance."

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