Duplicate publication is publication of an article that overlaps substantially with an article published elsewhere. Patterns of duplication are not well understood. To investigate duplication patterns and propose a decision tree for classification. We searched a comprehensive list of systematic reviews (1989 through August 15, 2002) in anesthesia and analgesia that is accessible on the Internet. We selected published full articles of duplicates that had been identified in these systematic reviews. Abstracts, letters, or book chapters were excluded. Authors of 56 (40%) of 141 systematic reviews acknowledged identification of duplicates. Duplication patterns were identified independently by all investigators comparing samples and outcomes of pairs of duplicates and main articles. Information on cross-reference, sponsorship, authorship, and publication characteristics was extracted from the articles. The 56 systematic reviews included 1131 main articles (129 337 subjects) and excluded 103 duplicates (12 589 subjects) that originated from 78 main articles. Sixty articles were published twice, 13 three times, 3 four times, and 2 five times. We identified 6 duplication patterns: (1A) identical samples and identical outcomes (21 pairs); (1B) same as 1A but several duplicates assembled (n = 16); (2) identical samples and different outcomes (n = 24); (3A) increasing sample and identical outcomes (n = 11); (3B) decreasing sample and identical outcomes (n = 11); (4) different samples and different outcomes (n = 20). The prevalence of covert duplicate articles (without a cross-reference to the main article) was 5.3% (65/1234). Of the duplicates, 34 (33%) were sponsored by the pharmaceutical industry, and 66 (64%) had authorship that differed partly or completely from the main article. The median journal impact factor was 1.8 (range, 0.1-29.5) for duplicates and 2.0 (range, 0.4-29.5) for main articles (P =.13). The median annual citation rate was 1.7 (range, 0-27) for duplicates and 2.1 (range, 0-31) for main articles (P =.45). The median number of authors was 4 (range, 1-14) for duplicates and 4 (range, 1-15) for corresponding main articles (P =.02). The median delay in publication between main articles and duplicates was 1 year (range, 0-7 years). Duplication goes beyond simple copying. Six distinct duplication patterns were identified after comparing study samples and outcomes of duplicates and corresponding main articles. Authorship was an unreliable criterion. Duplicates were published in journals with similar impact factors and were cited as frequently as main articles.
