If you would like to cite the same source more than once in a Wikipedia article, you should use named references.
Say, for example, that you have a source (George Clooney by John Doe) that verifies that George Clooney is an American actor. You want to also cite that same source to support that he won many awards. You can do so without writing out the full citation more than once:
Visual editing
editThe VisualEditor will name citations automatically when they are used more than once in an article. To reuse an existing citation, open the "Cite" popup from the VisualEditor toolbar, then click "Reuse". From there, you can select the reference you wish to repeat.
Source editing
editRefToolbar
editYou can use the citation toolbar ("RefToolbar") to name references. When you first create a reference, you can enter a value in the "Ref name" box. When you want to reuse this reference, click the "Named references" button on the citation toolbar and choose which reference you would like to insert.
Manual
editTo manually name a reference, add a "name" to the opening ref tag. This name can be anything you like; in this case, the reference is named "ClooneyArticle."
<ref name=ClooneyArticle>Doe, John. ''George Clooney''. Harvard University Press, 1971, p. 1.</ref>
Any time you want to use this reference again, insert it by simply adding the tag (notice the slash at the end of the reference tag):
<ref name=ClooneyArticle />