Well, that depends on the keyboard app you're using, to an extent. GBoard has a built-in clipboard that saves items for 1 hour. The standard clipboard keeps only one item so the next item you copy would overwrite it.
Or, the most simple solution - reboot. The standard clipboard is stored in what's called volatile memory, when the device is powered off that data is lost.
If your phone is not behaving in this way, we'll need more info about add-on clipboard apps you may have installed.
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