I've heard a number of people identify the two witnesses as Enoch and Elijah.

Their proof text for this appears to be:

Hebrew 9:27 And as it is appointed unto men once to die, but after this the judgment:

They say that since these hadn't died, that the two witness death would be their "one" time to die.

Enoch and Elijah were taken alive, without dying.

On the other hand, there had been others who had been dead / slain, and brought back to life.

I Kings 17:20 And he cried unto the LORD, and said, O LORD my God, hast thou also brought evil upon the widow with whom I sojourn, by slaying her son?

 21 And he stretched himself upon the child three times, and cried unto the LORD, and said, O LORD my God, I pray thee, let this child’s soul come into him again.

 22 And the LORD heard the voice of Elijah; and the soul of the child came into him again, and he revived.

The slain child had his soul returned to him and he revived.

II Kings 4:32 And when Elisha was come into the house, behold, the child was dead, and laid upon his bed.

 33 He went in therefore, and shut the door upon them twain, and prayed unto the LORD.

 34 And he went up, and lay upon the child, and put his mouth upon his mouth, and his eyes upon his eyes, and his hands upon his hands: and he stretched himself upon the child; and the flesh of the child waxed warm.

 35 Then he returned, and walked in the house to and fro; and went up, and stretched himself upon him: and the child sneezed seven times, and the child opened his eyes.

These two boys had died, and were brought back to life.

Hopefully, no one would dare teach that these two boys were the first fruits of the resurrection, rather than Jesus.

No, they were not resurrected with glorified bodies, to never die again.

Jesus' resurrection was like that, but not these two boys in the old testament.

There are no scriptures to indicate that these two boys were subsequently translated without dying again.

No, they died normal deaths, later on, they died twice.

Then, what about those in the coming blessed hope?

Those in the blessed hope do not experience "once to die".

But rather are changed and receive resurrection bodies while yet alive.

So, to say that "once to die" is a mandate, requiring that Enoch and Elijah must "die once", is not necessarily true.