Adam Clarke on Hebrews 9:27

As it is appointed - Αποκειται· It is laid before them by the Divine decree: Dust thou art, and unto dust thou shalt return. Unto men generally, during the course of the present world, not all men as some falsely quote; for Enoch and Elijah have not died, and those that shall be alive at the day of judgment shall not die, but be changed.

But after this the judgment - They shall die but once, and be judged but once, therefore there is no metempsychosis, no transmigration from body to body; judgment succeeds to dying; and as they shall be judged but once, they can die but once.