Sharon in the comments said: I think it means a person that becomes a Christian will only die once (physically), and not spiritually, but someone who is never born again will die both physically and spiritual

Yes.

Because I saved person only dies once Because after their physical death comes heaven which is life.

But the unsaved person Dies twice. Once physically, and then hell which is eternal death.

My dad was telling me and my sister's that he had heard this on his way to work from the radio. And I thought how cool! (this would make a great blog post.)