Brother Fred, I don't like being cantankerous, but...
How does the fact that Enoch and Elijah never died, but God took them go with what you are saying - that people will die one and only one time. How does the fact that millions of Christians will be alive when Jesus returns and they will NEVER die the first time go with what you are saying.
With those examples in mind, I don't think Hebrew 9:27 is saying that every human being will die once and only once. It's not teaching a dogma about death. It's teaching about Christ's one-for-all sacrifice.
Here it is in context.
This is a teaching about Jesus. The high priest had to seek God for the people's atonement every single year. Jesus Christ paid the atoning price only once. Paul interjects a statement about humanity - that just like humanity will die and then be judged at the end - Jesus has to die and will be the Judge.
There isn't a numerical or quantitative idea here. And the exceptions to dying once - Enoch, Elijah, those alive at Christ's return, those raised from the dead to die again don't negate the intent of the verse. Just as it is the normal thing that we die and then we will face God at His judgment seat, Jesus died and IS mediating for us now and WILL be part of that judgment.