You need to differentiate between the sources by putting a, b, c, etc. after the year in both your in-text citations and and reference list entries, e.g.
In-text citation
(Ogunyemi, 2012a)
(Ogunyemi, 2012b)
Reference list
Ogunyemi, O. (2012a) Sourcing and representation routines in the black African press in the United Kingdom. In: I. Rigoni and E. Saitta (eds.) Mediating cultural diversity in a globalised public space. Basingstoke: Palgrave Macmillan, 153-168.
Ogunyemi, O. (2012b) What newspapers, films, and television do Africans living in Britain see and read?: the media of the African diaspora. Lampeter: Edwin Mellen Press.
