1. Whom did you laugh at?
2. At whom did you laugh?
In the first, "at" stands at the end of the sentence and in the second,"at" is with the [STRIKE]preposition[/STRIKE] pronoun.
[STRIKE]Are[/STRIKE] Do these sentences mean the same?
Yes, they mean the same.
In case my marks aren't clear, you need to put a space after the full stop after the numbers 1 and 2.
Don't ask "Are these sentences the same?" They are clearly not the same (that means identical) because the words are in a different order.
You are right, but it is not right to use the unnatural passive form.
"Who was laughed at by you?" is passive voice of "Whom did you laugh at?" then, can we write the passive voice " At who was laughed by you?" and I think it is not the unnatural passive form....