I have two multi-author references that have the same senior author, but differ in the junior authors; BibTeX renders this citation in the text as 'Bloggs et al. (1999,1999)'. It should of course display 'Bloggs et al. (1999a,b)'.
How can I force it to append letter suffixes to the date when I have multiple 'Bloggs et. al.' references published in the same year?