Does common knowledge have to be referenced?

Common knowledge doesn't have to be cited; but if you know the source it doesn't hurt to cite it as the definition of common knowledge is subjective.

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It's not a good idea to overload your work with references to things that are 'common knowledge' and undisputed. It can be irritating if there are, for example, two references per line in an essay or paper, and it may even give readers the impression that you are nervous about the possibilty of being faulted for lack of references.