Duplicate publication can introduce substantial biases if studies are inadvertently included more than once in a meta-analysis (Tramèr 1997). Duplicate publication can take various forms, ranging from identical manuscripts to reports describing different numbers of participants and different outcomes (von Elm 2004). It can be difficult to detect duplicate publication, and some ‘detective work’ by the review authors may be required.
Some of the most useful criteria for comparing reports are:
author names (most duplicate reports have authors in common, although it is not always the case);
location and setting (particularly if institutions, such as hospitals, are named);
specific details of the interventions (e.g. dose, frequency);
numbers of participants and baseline data; and
date and duration of the study (which can also clarify whether different sample sizes are due to different periods of recruitment).
Where uncertainties remain after considering these and other factors, it may be necessary to correspond with the authors of the reports.