Twice dead are mentioned in Jude:12.
These are spots in your feasts of charity, when they feast with you, feeding themselves without fear: clouds they are without water, carried about of winds; trees whose fruit withereth, without fruit, twice dead, plucked up by the roots; — KJV, Jude 1:12
But from [22-23] we can see that at least some of them can be eternally saved.
 And of some have compassion, making a difference:
 And others save with fear, pulling them out of the fire; hating even the garment spotted by the flesh.— KJV, Jude 1:22-23
Though its not clear that everyone can be eternally saved because of God can cast off someone forever.
And thou, Solomon my son, know thou the God of thy father, and serve him with a perfect heart and with a willing mind: for the LORD searcheth all hearts, and understandeth all the imaginations of the thoughts: if thou seek him, he will be found of thee; but if thou forsake him, he will cast thee off for ever. — KJV, I Chronicles 28:9
How should we deal with the twice dead ?
Of some have compassion making a difference and other save with fear, pulling them out of the fire hating even the garment spotted by the flesh.