If you are referring to two sources by the same author, produced in the same year, you can distinguish between them by adding letters to the end of the year for both your citation and reference.
Example: Hoskin, B. (2011a) Cocoa growing: implications for sustained development. Journal of Sustainable Growth. 39(5) pp.44-51.
Hoskin, B. (2011b) Global warming effects on cocoa crops. Journal of Sustainable Growth. 4(5) pp.94-101.