This is usually the result of two different references being used in the manuscript with the same last name. Endnote interprets them either correctly (say Mark Murphy and Joe Murphy) or incorrectly (say it is entered in one as Murphy, Mark and in the other as Murphy, M.) as different authors. Many publications dictate that different authors with the same surname be distinguished and this is a setting in the output style Citations>Author Name options. When it IS the same author, the easiest way to avoid this problem is to copy the author from one record to replace it in the other. Sometimes the difference isn’t all that obvious.