How can one properly cite a source that has already been cited in a research paper?

To properly cite a source that has already been cited in a research paper, you should use the original source that was cited in the paper you are reading. This is known as citing a secondary source. In your citation, you should include the author and publication information of the original source, followed by "as cited in" and the author and publication information of the paper you are reading. This helps give credit to the original source while acknowledging the paper where you found the information.