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This happens when you skip a step in the instructions for Discovery#EnablingDiscovery. In particular, forgetting to set recent.submissions.count = 0 will mean that recent items continue to be rendered by the Browse aspect in addition to the Discovery aspect, resulting in duplication.
To fix this, set recent.submissions.count = 0 in [dspace]/config/dspace.cfg and restart your servlet container. If needed, also clear the Cocoon cache: TechnicalFaq#TechnicalFaq-ClearingCocoon(XMLUI)cache