diff --git a/js/entity_browser.common.js b/js/entity_browser.common.js
index d28e176..c50bb63 100644
--- a/js/entity_browser.common.js
+++ b/js/entity_browser.common.js
@@ -99,11 +99,11 @@
     }
     else if (selection_mode === 'selection_prepend') {
       // Prepend selected entities to existing list of entities.
-      combined_entities = selected_entities.concat(existing_entities);
+      combined_entities = [...new Set([...selected_entities, ...existing_entities])];
     }
     else {
       // Append selected entities to existing list of entities.
-      combined_entities = existing_entities.concat(selected_entities);
+      combined_entities = [...new Set([...existing_entities, ...selected_entities])];
     }
 
     // Having more elements than cardinality should never happen, because