Index: addon/frontend/src/main/java/org/onehippo/taxonomy/plugin/model/JcrTaxonomy.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- addon/frontend/src/main/java/org/onehippo/taxonomy/plugin/model/JcrTaxonomy.java	(revision 57515)
+++ addon/frontend/src/main/java/org/onehippo/taxonomy/plugin/model/JcrTaxonomy.java	(revision )
@@ -1,5 +1,5 @@
 /*
- *  Copyright 2009-2015 Hippo B.V. (http://www.onehippo.com)
+ *  Copyright 2009-2016 Hippo B.V. (http://www.onehippo.com)
  *
  *  Licensed under the Apache License, Version 2.0 (the "License");
  *  you may not use this file except in compliance with the License.
@@ -177,6 +177,17 @@
         }
     }
 
+    protected void loadAllCategories(final Node node) {
+        if (categories == null) {
+            categories = new TreeMap<>();
+            try {
+                loadCategories(node);
+            } catch (RepositoryException ex) {
+                log.error("failed to load categories", ex);
+            }
+        }
+    }
+
     private void loadCategories(final Node node) throws RepositoryException {
 
         if (node instanceof HippoNode) {
@@ -233,5 +244,8 @@
         return getNodeModel().hashCode() ^ 991;
     }
 
+    public Map<String, JcrCategory> getCategoriesMap() {
+        return categories;
+    }
 
 }
Index: addon/frontend/src/main/java/org/onehippo/taxonomy/plugin/model/TaxonomyObject.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- addon/frontend/src/main/java/org/onehippo/taxonomy/plugin/model/TaxonomyObject.java	(revision 57515)
+++ addon/frontend/src/main/java/org/onehippo/taxonomy/plugin/model/TaxonomyObject.java	(revision )
@@ -1,5 +1,5 @@
 /*
- *  Copyright 2009-2015 Hippo B.V. (http://www.onehippo.com)
+ *  Copyright 2009-2016 Hippo B.V. (http://www.onehippo.com)
  *
  *  Licensed under the Apache License, Version 2.0 (the "License");
  *  you may not use this file except in compliance with the License.
@@ -16,6 +16,8 @@
 package org.onehippo.taxonomy.plugin.model;
 
 import java.util.Iterator;
+import java.util.Map;
+import java.util.TreeMap;
 
 import javax.jcr.ItemNotFoundException;
 import javax.jcr.Node;
@@ -29,6 +31,7 @@
 import org.hippoecm.frontend.model.ocm.JcrObject;
 import org.hippoecm.repository.api.HippoNodeType;
 import org.hippoecm.repository.api.HippoSession;
+import org.hippoecm.repository.util.NodeIterable;
 import org.onehippo.taxonomy.api.TaxonomyNodeTypes;
 import org.onehippo.taxonomy.plugin.ITaxonomyService;
 import org.onehippo.taxonomy.plugin.api.JcrCategoryFilter;
@@ -46,6 +49,8 @@
 
     private final ITaxonomyService taxonomyService;
 
+    private Map<String, JcrCategory> allCategories;
+
     /**
      * Constructor
      */
@@ -119,7 +124,7 @@
             taxonomyNode = taxonomyNode.getParent();
         }
         JcrTaxonomy taxonomy = toTaxonomy(new JcrNodeModel(taxonomyNode), true);
-        if (taxonomy.getCategoryByKey(key) != null) {
+        if (taxonomy.getCategoryByKey(key) != null || categoryKeyExists(key)) {
             throw new TaxonomyException("Key " + key + " already exists");
         }
 
@@ -137,6 +142,46 @@
         }
 
         return toCategory(new JcrNodeModel(child), true);
+    }
+
+    //Checks if a key exists in any existing taxonomy
+    private boolean categoryKeyExists(String key) throws RepositoryException {
+        if (allCategories == null) {
+            allCategories = new TreeMap<>();
+        }
+
+        loadAllCategories();
+
+        JcrCategory category = allCategories.get(key);
+        if (category != null) {
+            Node node;
+            try {
+                node = category.getNode();
+                if (node != null && node.getSession().itemExists(node.getPath())) {
+                    return true;
+                }
+            } catch (RepositoryException e) {
+                log.error("failed to find category by key " + key, e);
+            }
+        }
+        return false;
+    }
+
+    private void loadAllCategories() throws RepositoryException {
+        JcrNodeModel jcrNodeModel = this.getNodeModel();
+        if (jcrNodeModel == null || jcrNodeModel.getNode() == null) {
+            return;
+        }
+        while (jcrNodeModel.getParentModel() != null && !jcrNodeModel.getNode().getPrimaryNodeType().getName().equals("hippotaxonomy:container")) {
+            jcrNodeModel = jcrNodeModel.getParentModel();
+        }
+        for (Node handle : new NodeIterable(jcrNodeModel.getNode().getNodes())) {
+            for (Node taxonomyNode : new NodeIterable(handle.getNodes())) {
+                JcrTaxonomy taxonomy = toTaxonomy(new JcrNodeModel(taxonomyNode), true);
+                taxonomy.loadAllCategories(taxonomyNode);
+                allCategories.putAll(taxonomy.getCategoriesMap());
+            }
+        }
     }
 
     // Factory methods to subclass, e.g. to customize node creation.