Citation, Documentation of Sources

Q. If it becomes necessary to use two editions of the same title, do both editions need to be included in the bibliography?

A. If you cite both editions or rely on both editions for data, then yes, they should both be listed in your bibliography. If the two editions have different authors or different titles, or both, it’s best to list them separately:

Fowler, H. W. A Dictionary of Modern English Usage. 2nd ed. Revised and edited by Sir Ernest Gowers. Oxford: Oxford University Press, 1965.

———. Fowler’s Dictionary of Modern English Usage. 4th ed. Edited by Jeremy Butterfield. New York: Oxford University Press, 2015.

If the details are substantially the same for both editions, you may list them under a single entry:

University of Chicago Press. The Chicago Manual of Style. 16th and 17th eds. Chicago: University of Chicago Press, 2010 and 2017.

For a detailed discussion of the 3-em dash in the second example above, including some caveats, see CMOS 14.67–71.