A writer asked me today about how the stuff written for the introduction had turned out a lot like the stuff written for the abstract. This seems to me to be about right.
Politicians talk about staying on message--which typically means repeating the same simplistic argument over and over. As writers we don't want to talk to our readers as if they were fools, so we want to be careful about repeating ourselves too much, but that being said, it's also important to stay on message--to keep saying why you're writing and how the work relates to the main questions that you want to address. To some extent there is a level of redundancy built in, but on another level, it's not so much redundancy, as a rephrasing of an old idea in a new context--because each time you are returning to a large guiding idea, you are doing so in the context of a different section of the written work, and therefore, sometimes you are talking about the main idea in the context of one idea, or one set of data, or one time period, etc., and sometimes you are talking about it in the context of a different idea. Each time you mention the main point--each time you are "staying on message"--you are rephrasing it with different linkages, so that you are reminding the reader that what you're doing is looking at the new idea in the context of the main message.
And there is no clear guidance on exactly when to do this, or how much to stay on message. You want to make sure that your reader can easily follow the main point, so that their energy isn't lost trying to stay on track and they can follow the subtleties of the argument. At the same time you want to make sure that you're not repeating yourself so much that the reader feels like you're talking down to them. And the right balance of this will be different for different readers. Nonetheless, you're looking for some balance of this, and therefore your choices will seem fraught with difficulties: "did I do too much?", "did I do too little?" When working with a point of balance, one can (obviously) fall off on either side at any moment.