Does not the meaning of the noble verse: "He said: 'As for him who wrongs, we will punish him, then he will be returned to his Lord, and He will punish him with a grievous torment,'" signify that the servant will be punished in this world and the hereafter, and Allah does not punish a servant for a sin twice?

There is no ambiguity in the verse, for the words of Dhul-Qarnayn are an excerpt from his speech. He threatened with death those who persisted in injustice through disbelief. Then, this persistent one will be returned to his Lord, who will punish him in the Hereafter. So, if a disbeliever is killed, the killing does not protect him from the torment of the Hereafter. Allah Almighty says: "Indeed, he who associates others with Allah - Allah has forbidden him Paradise, and his refuge is the Fire. And for the wrongdoers there are no helpers" and Allah Almighty says: "Indeed, the criminals will be in the torment of Hell, abiding eternally therein." As for a Muslim who commits a sin that necessitates a prescribed legal punishment and is punished for it in this world, that punishment will be an expiation for him, and he will not be punished for it in the Hereafter, according to the preponderant opinion, based on the hadith (definition): "And whoever commits any of those things and is punished for it, it will be an expiation for him."