This paper analyses the phenomenon when a publication referring to the oeuvre of a research group (i.e. all the articles published by its members) cites several articles rather than one article from that oeuvre (multiple citations, MC). It is shown that significant differences exist between research groups with respect to the frequency at which MC to their respective oeuvres occur, and that these differences affect to some extent rankings of these groups based on citation counts. In order to find an explanation for our results, four factors are discussed: (1) the impact of a research group; (2) mutual multiple citing arrangements; (3) the size of a group's oeuvre and (4): the degree of common intellectual interest between the research activities in a group. No definite conclusions can be drawn yet on the extent to which these factors are responsible for the observed patterns in the MC frequency. We conclude however that attempts to identify top or sub-top groups in comparative evaluations based on citation analysis should be performed with the greatest care.