Background: Duplex appendix is very rare (incidence 0.004%) but has clinical relevance given the frequency of appendicectomies performed. Failure to recognise duplication can result in failure of treatment and medico-legal consequences. A review of published cases was undertaken to identify factors that may help in managing this rare condition. Methods: All English and non-English publications were identified in PubMed, Embase and Cochrane databases. Patient demographics, intraoperative findings, anatomical details and histopathology were analysed. Results: A total of 141 cases were identified [male/female ratio 1.4:1, median age 20 years (range foetus to 69 years)]. Duplication of the appendix ranges from branching of the appendix trunk to a fully matured appendix located elsewhere along the colon. Most can be categorised by the Cave–Wallbridge classification. There were 22 Type A, 8 Type B1, 46 Type B2 and 10 Type C cases (Cave–Wallbridge). There were six cases of horseshoe and two cases of triple appendix. Six reports gave anatomical descriptions that could not be classified by Cave–Wallbridge categories and in 11 cases there were no anatomical descriptions. Conclusions: An anteriorly placed appendix, away from the convergence of the taenia, or a normal appendix in the presence of convincing clinical or radiological signs of appendicitis should instigate a careful examination of the caecal pole and possible exploration of the retrocaecal space for appendiceal duplication. If the patient had previous surgery for congenital abnormalities, Type B1 or Type C duplication should be considered.