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You should always cite sources. Unless there are only two authors or disambiguation is needed, always collapse all authors except the first author by using the et al. abbreviation (e.g., James et al., 2008).
Sometimes you will encounter a source with many authors. Unless disambiguation is needed, collapse the citation from the very first time you cite it. For example, imagine that Jackson and three colleagues wrote an article in 1998. Even if you are citing them for the first time, you would still use the et al. abbreviation (e.g., Jackson et al., 1998).
Sometimes the same authors have written multiple papers in the same year. Imagine that you want to cite both Albert, Benjamin, Cox, and Daniels (2001) and Albert, Daniels, Cox, and Benjamin (2001). You can cite the first paper as (Albert, Benjamin, et al., 2001) and the second paper as (Albert, Daniels, et al., 2001) to disambiguate them. But what if the author list is exactly the same across two papers? For example, what if James and Taylor wrote two papers in 1987, and you want to cite both of them? You'll cite one paper as (James & Taylor, 1987a) and the other as (James & Taylor, 1987b) to disambiguate them (the paper with the alphabetically first title will be 1987a and the other one will be 1987b).
It is often useful to cite multiple papers for the same sentence. If you do so, the citations should always be listed alphabetically by the authors' names (e.g., Benjamin et al., 2004; Cox et al., 1984; Yellow et al., 2011).
Finally, if the exact same authors (in the exact same order) have written multiple papers, there is a shorthand notation. For example, you can cite Yip & Hallow (2001a, 2001b) and Yip & Hallow (2011) as (Yip & Hallow, 2001a, 2001b, 2011).