This is usually because you have cited two different articles by that author and the two are not identical. Little things like full stops, or spacing or one has full first name will trigger Endnote to comply with publishers to distinguish what it thinks are two different authors with the same surname. You should fix this by pasting the author from one record to the other (which is preferred).
for alternative “cludges” See here: http://endnote.com/kb/82224 – but the first two will mean that if you DO have two authors with the same Surname, that should be distinguished, you will not be using the correct format for the publication.