Die before you die. There is no chance after. ~ C.S. Lewis
Obviously, Lewis is using the word "die' in two different ways here. It would be impossible to die in the same way before you die again ... you'd already be dead.
There is a dying to self that Lewis is talking about. We must die to our own fallen thoughts, desires, plans, purposes, etc. And if we do not, then when we die on our last day, that death will be final.
Jesus taught about this in John 12:24 when he likes it to a seed falling to the ground and 'dies' to itself, that it may bear fruit. A seed that never experiences such a death eventually one day dies ... alone.
Dying to ourselves is a gift that God gives to us. This is not a pull yourselves up (or down as the case may be) by the boot straps accomplishment that we do by our own initiative and strength. It is a divine gift, to see our sin, to repent and die to it, and to find new life in the life that Christ gives us by grace.
Those who die the first death that Lewis speaks of have nothing to fear from the second death that all men will one day experience.