Where to put a cited paper: introduction or discussion? I have finished the first draft of my paper. It extends a statistical method and it is applied to an application. I cite many papers in the introduction. As a result, not so many left for the discussion part. If I put them here again, it seems redundant. So my question is: when you see a relevant paper, how do you know that it should be in the introduction or discussion?
Some very general guidelines from my own expertise; kept rather simple to be a starting point for first-timers in article writing:


In the Introduction one mentions/describes the papers most relevant to the research conducted. E.g., if one analyses the volume of wolves' howling, there should appear sentences like "in [1], Smith et al. measured the howling loudness of 12 wolves in the Carpathians, and found they are quite loud"
In the Methods section, one cites papers that are relevant to the methodology of the work/its analysis, e.g. something along the lines "following [2], we set up 18 automatic sound recorders [3] in the Beskids. These recorders were triggered according to the Wolf-Howler criterion [4], i.e..." blah-blah-blah "we performed standard statistical tests [5] to verify..."
In the Analysis/Results section, one usually mostly emphasizes what is new in the presented paper. Some immediate comparisons are very helpful, e.g. "the howling that we recorded is statistically louder than what was presented in [6]; this discrepancy may be due to the fact that most of the wolves that we recorded were males, while 17 out of 24 individuals measured by Hound &amp; Forest in [6] were females"...
The Discussion is the part where one discusses the importance of the presented research. One compares, emphasizing and discussing in more detail, similarities/differences of the newest results with previous findings. If a new method is applied, one discusses its strength and weaknesses in the context of the present and possible future research. Phase like this can also occur: "We showed that the automatic recording of wolves howling is efficient, and so can be applied in the field of raccoon noises [7]...".


I think that in my papers, papers that were not mentioned earlier rarely appear in the Discussion. Although there are exceptions, e.g. when some comment from a paper from a different field seems to be adequate in the light of the discussed results.

Overall, I think that most (and all relevant) papers should be cited as early in the manuscript as possible. There are no general rules of thumb whether it's fine to introduce a paper in the Discussion or not. It's mostly about the gut feeling and personal writing style. I can imagine people justifying that one should absolutely not cite in the Discussion/Conslusion/Summary papers that weren't cited earlier in the text. They may have good arguments, certainly, and if one tends to agree, it's completely fine. There might be exceptions, though, that are subject to personal judgment and one's preferences, so I'll stop here.