Negative questions, in other words ones where the inverted auxiliary is negated tend to create an implicature that the speaker already has an opinion about the probable answer. We can consider them 'biased' in this respect. The following question, for example, may be taken to indicate that the speaker suspects (or has suspected until very recently) that Mary gave the book to the listener yesterday:
- Didn't Mary give you the book yesterday?
In the Original Poster's example, the negation of the auxiliary is is used to convey that the speaker suspects (or suspected until just now) that the proposition in the rest of the clause is the case. It so happens in this example that the proposition that the speaker suspects is true is a negative one:
Notice that simple single negation in yes/no questions does not change the core meaning of a question at all:
There are no situations where the answer to the first question will be different from the answer to the second question. For this reason, there is no sense in which double negation can cancel itself out in questions. In normal declarative sentences, negation changes the actual core meaning of the sentence:
- She gave me the book.
- She didn't give me the book.
If we add further negation, the truth conditional meaning of the sentence will be the same as the non-negated sentence:
- She didn't not give me the book.
But, as we have seen there is no real negation in questions to be cancelled out by further negation. However, a doubly negated question and a doubly negated declarative sentence have something in common. They both indicate that there is some negative proposition which has been previously entertained. In the Original Poster's example, it is the proposition that there is no need. In the book example above it is the idea that she didn't give me the book.
In answer to The Original Poster's question then, the two negations in the question do not cancel each other out in any way. Arguably the different negations have different functions. One is there to indicate some previously supposed idea, the other is to show that the idea is negative.
There is in fact no other way to do this apart from using double negation. Very often the non-truth conditional meaning of a sentence is as communicatively important as the truth-conditional meaning. This is the case with the Original Poster's example. The communication of the presupposition on the part of the speaker that there is no need is part of the suasive force of the question.