You are warned against committing the same mistake again.

The correct answer is D. No improvement.

The sentence "You are warned against committing the same mistake again" is grammatically correct and conveys the intended meaning clearly. The preposition "against" is used to express a warning or prohibition. In this case, the speaker is warning the listener not to commit the same mistake again.

The other options are incorrect because they do not convey the intended meaning as clearly. Option A, "to commit," would suggest that the listener is being instructed to commit the same mistake again. Option B, "for committing," would suggest that the listener is being blamed for committing the same mistake. Option C, "against to commit," is grammatically incorrect.

Therefore, the correct answer is D. No improvement.