please tell if the sentence is correct?

The principal apologised to the students, admitting that he had failed in his duty in having failed failing to prevent the attack on the institution premises.

But he has already mentioned "he failed in his duty"...Why is "failed" used again?
They have already failed in their duty to prevent the attack.

It is certainly so, for fivejedjon tries to be strict to your original sentence, or a simple lack of attention, I think. That your sentence makes sense for him is another option:)
.....................he had failed in his duty to prevent\ in preventing the attack.....

daylightttIt is certainly so, for fivejedjon tries to be strict to your original sentence, or a simple lack of attention, I think. That your sentence makes sense for him is another option:).....................he had failed in his duty to prevent\ in preventing the attack.....

The principal apologised to the studen

I see.
In fact, I weren't sure whether or not I could repeat "in" consecutively. To be honest, I have done such a thing for the first time. Thanks, fivejedjon. Your opinion always has a great value for me.

I think it has to be "I wasn't sure" and not weren't.

Well, sure! It is a kind of mistake done unknowingly. ( I really like using it) Of course, we can use the "weren't" with conditional clauses and wish clauses.

Sorry, not done but made:) ......drowsiness!