Citing the same idea in different studies Say like you have an idea that says: 'X is a cheap alternative to Y' (Smith 2010, p. 2). Now you write a paper about ABC and paraphrase Smith's idea. Next week, you write another paper which happens to have an idea similar to ABC and you had to use Smith's idea again to solidify your choice or analysis. The same thing happens for a third time, etc... Can you use it again over and over? Albeit I will admit that I try to differently paraphrase the idea each time I use it.
Of course! If you write multiple papers in the same area, you'll cite many of the same papers in each paper. No one should be required to know your history of publication to get a fully account of related work from one of your papers.