in eternity, though, we are eternally redeemed... meaning, once we are redeemed ion full upon Christ's return, it is eternal, never to be undone, never to be changed.
now, if we take THAT definition, and apply it to Matthew 25:46, there is no contradiction in scripture... the punishment for sin has always been death, Romans 6:23. the punishment that Matthew 25:46 refers to is not living forever in torment, but death... so when it says "eternal punishment", it is referring to the end result of being thrown into the lake of fire... death. a death that cannot be undone nor changed.