If you're a Christian, you will only die once.
At the end of encouraging the church at Smyrna, Jesus says,
He who has an ear, let him hear what the Spirit says to the churches. The one who conquers will not be hurt by the second death. (Revelation 2:11)
The first part of the verse is the same as how Jesus concluded His letter to the church at Ephesus. But this time, the second sentence is different. He mentions, again, "the one who conquers," which refers to those who overcome the trials of this life. Jesus is the first conqueror. He said that in this world, we will have trouble but calls us to take heart; He has overcome the world (John 16:33). And in Christ, we are more than conquerors (Romans 8:37).
All those who have believed in Jesus Christ and have been justified by faith in Him will conquer, and the one who conquers, Jesus says, "will not be hurt by the second death."
Later in chapter 20, verse 14, the "second death" is described as the lake of fire. This is the punishment that still awaits those who have persisted in their rejection of God. It is eternal separation from His kindness and mercy. Note that it isn't a physical death, but a spiritual one. Those who will experience the second death have already died the first death, a physical one, but their souls have lived on, awaiting final punishment.
Matthew 25:46 describes hell as a place of punishment, not a place of destruction. Furthermore, Jesus calls it "eternal" punishment in that verse. The point we're making here is that the second death is not annihilation. As much as it might comfort us to think that someone's punishment is not eternal, that's not what the Bible teaches; that's not what Jesus has taught. The punishment for sinning against God and rejecting the Savior whom He has sent is a death that is experienced eternally.
We don't relish this idea. Even God says of Himself in Ezekiel 18:23 that He doesn't take pleasure in the death of the wicked. We will rejoice in God's glorious justice, but we don't wish hell upon anyone.
Instead, knowing that this punishment awaits all those who remain in their sin and reject Christ, we should be all the more diligent to proclaim to everyone that God has provided a way out--Jesus Christ Himself. So, preach that boldly.
And if you're reading this as someone who does not believe in Jesus Christ, we proclaim to you now that if you continue down that path, you will suffer this second death that Jesus is talking about...spiritual death that lasts forever and ever. But if you put your faith in Him, then he will forgive you of all your sins and you will have eternal life instead.
Christians who are suffering, the main audience of what Jesus is saying here in Revelation, be encouraged. You may "be faithful unto death" (v. 10), but you "will not be hurt by the second death" (v. 11). The worst anyone can do to you is send you to heaven and make you a partaker of that which you desire most (Richard Sibbes). Take heart, conqueror. Christ has overcome this world for you.