When multiple references have an identical author (or authors) and publication year, include a lowercase letter after the year. These letters are assigned when the references are placed in order in the reference list (alphabetically by title, following the guidelines in Section 9.47).
The year–letter combination is used in both the in-text citation and the reference list entry. Use only the year with a letter in the in-text citation, even if the reference list entry contains a more specific date.
(Blondaux & O’Hanrahan, 2018a)
Blondaux and O’Hanrahan (2018b)
(Dreschke, n.d.-a, n.d.-b)