In your phone's settings, you will find unique, unchangeable hardware numbers such as the serial number, Wi-Fi Mac address, Bluetooth Mac address and IMEI. Make a note of these numbers. This way, you can always be sure that your device has not been secretly replaced.
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Make sure that you store these numbers in a secret place. It is best to store them in encrypted form. This way, you don't have to worry that these numbers will be used in the future to assign a specific phone to you.