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Hmm good question! I've wondered about it alot myself, from reading other papers I think it is largely determined by the writing style. On the most part though (so long as the sentences are linked) I think you can put in one reference. -Assuming you mean what I think you mean. As in, you have one paper that you want to reference and you want to talk about it over a series of sentences but not sure if you have to stick the reference in every sentence.
Let's say you have a passage of several sentences the first sentence of which makes clear that someone else's work or commentary is being referenced. If the entire passage -- of paraphrase, perhaps with various short quotations sprinkled in -- is clearly cohesive, then it can make best sense to place a single citation at the concluding sentence. Even cleaner is if this sentence concludes also the paragraph. This works best, of course, if there is a single page or page range, or perhaps only a couple of page ranges. If the citations skip all over the place, it might be cleaner to place those disparate pages at the end of the relevant sentences. If, on the other hand, the several sentences do not comprise one clearly cohesive citation -- if different aspects of the work or different points of view or remarks from within by different individuals are being compared and contrasted, for example -- it may be better for each shift of direction or perspective to sport its own individual citation, generally at the end of the sentence in question. Likewise, if commentary or analysis on the part of the paper writer is interspersed with the source material, it may be clearer if the citations are used throughout the passage to clearly delineate one from the other. There's a bit of an art to citation, I think, once the material and its treatment become more complex. If the end result is clean and clear, such that the reader is never left in doubt as to who is the original author of the thoughts in any given sentence, and as long as there are no other violations of citation style, the solution the paper writer has arrived at should be fine. That's my take on it.
It really took me too long to learn that "There's a bit of an art to citation", I used to think there were fixed rules to everything only now am I really realising that how you cite depends on your paper. I like your answer Redwood_Girl :)