Use lower case letters (a, b, etc.) to distinguish between works published in the same year by the same author(s).
Koriat (2008a) states that… this was supported by Koriat (2008b) …
The suffixes are assigned in the reference list, where these kind of references are ordered alphabetically by title (of the article, chapter, or complete work).
Koriat, A. (2008a). Easy comes, easy goes? The link between learning and remembering and its exploitation in metacognition. Memory & Cognition, 36(2), 416-428. https://doi.org/10.3758/MC.36.2.416
Koriat, A. (2008b). Subjective confidence in one's answers: The consensuality principle. Journal of Experimental Psychology: Learning, Memory, and Cognition, 34(4), 945-959. https://doi.org/10.1037/0278-73188.8.131.525