Citing a source of a figure in a text
It is normally enough to cite it once. If it is clear to the reader there should be no issue. The caption is complete, with the citation. The text itself can read more naturally. 

But this is just a style suggestion. Correctness and clarity are the goals. 

Caveat: Some journal may have their own style guide, but you will be told of that as part of the review process. For a dissertation it is fine to say it once unless your advisor objects.