Use lower case letters (a, b, etc.) to distinguish between works published in the same year by the same author(s).
Koriat (2018a) states that… this was supported by Koriat (2018b) …
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. (2018a). Agency attributions of mental effort during self-regulated learning. Memory & Cognition, 46(3), 370–383. https://doi.org/10.3758/s13421-017-0771-7
Koriat, A. (2018b). When reality is out of focus: Can people tell whether their beliefs and judgments are correct or wrong?. Journal of Experimental Psychology: General, 147(5), 613-631. https://doi.org/10.1037/xge0000397