If you want to cite a work which is referenced in another work, you should try and track down the original. However, if this isn’t possible, make it clear in your text where you found the information and only include a reference to the document you’ve read.
Example: According to Dunn (as cited in Campbell & Muncer, 2020), the theory was discredited…
Your reference list will include the full details of the Campbell and Muncer work, but no mention of Dunn’s.