When referencing publications by a single author in the same year they need to be distinguished by applying a lower case letter after the year as seen below

Author, Initials, Year followed by a letter. Title of publication. Place: Publisher.

Goodfrey, B., 1996a. Opinions are great. Chatham: University of Kent

Goodfrey, B., 1996b. Opinions Developed. Chatham: University of Kent