diff --git a/lucene/CHANGES.txt b/lucene/CHANGES.txt
index 27ad0249c61..2815a072145 100644
--- a/lucene/CHANGES.txt
+++ b/lucene/CHANGES.txt
@@ -291,6 +291,10 @@ Bug Fixes
 * LUCENE-4608: Handle large number of requested fragments better.
   (Martijn van Groningen)
 
+* LUCENE-4633: DirectoryTaxonomyWriter.replaceTaxonomy did not refresh its
+  internal reader, which could cause an existing category to be added twice.
+  (Shai Erera)
+  
 Changes in Runtime Behavior
   
 * LUCENE-4586: Change default ResultMode of FacetRequest to PER_NODE_IN_TREE.
diff --git a/lucene/facet/src/java/org/apache/lucene/facet/taxonomy/directory/DirectoryTaxonomyWriter.java b/lucene/facet/src/java/org/apache/lucene/facet/taxonomy/directory/DirectoryTaxonomyWriter.java
index f8be8c78682..16bf5206a46 100644
--- a/lucene/facet/src/java/org/apache/lucene/facet/taxonomy/directory/DirectoryTaxonomyWriter.java
+++ b/lucene/facet/src/java/org/apache/lucene/facet/taxonomy/directory/DirectoryTaxonomyWriter.java
@@ -1027,6 +1027,8 @@ public class DirectoryTaxonomyWriter implements TaxonomyWriter {
     indexWriter.deleteAll();
     indexWriter.addIndexes(taxoDir);
     shouldRefreshReaderManager = true;
+    initReaderManager(); // ensure that it's initialized
+    refreshReaderManager();
     nextID = indexWriter.maxDoc();
     
     // need to clear the cache, so that addCategory won't accidentally return
diff --git a/lucene/facet/src/test/org/apache/lucene/facet/taxonomy/directory/TestDirectoryTaxonomyWriter.java b/lucene/facet/src/test/org/apache/lucene/facet/taxonomy/directory/TestDirectoryTaxonomyWriter.java
index f696b6e0737..1beeb8ce8c2 100644
--- a/lucene/facet/src/test/org/apache/lucene/facet/taxonomy/directory/TestDirectoryTaxonomyWriter.java
+++ b/lucene/facet/src/test/org/apache/lucene/facet/taxonomy/directory/TestDirectoryTaxonomyWriter.java
@@ -9,6 +9,7 @@ import java.util.concurrent.atomic.AtomicInteger;
 
 import org.apache.lucene.facet.taxonomy.CategoryPath;
 import org.apache.lucene.facet.taxonomy.TaxonomyReader;
+import org.apache.lucene.facet.taxonomy.directory.DirectoryTaxonomyWriter.MemoryOrdinalMap;
 import org.apache.lucene.facet.taxonomy.writercache.TaxonomyWriterCache;
 import org.apache.lucene.facet.taxonomy.writercache.cl2o.Cl2oTaxonomyWriterCache;
 import org.apache.lucene.facet.taxonomy.writercache.lru.LruTaxonomyWriterCache;
@@ -316,12 +317,12 @@ public class TestDirectoryTaxonomyWriter extends LuceneTestCase {
   public void testReplaceTaxonomy() throws Exception {
     Directory input = newDirectory();
     DirectoryTaxonomyWriter taxoWriter = new DirectoryTaxonomyWriter(input);
-    taxoWriter.addCategory(new CategoryPath("a"));
+    int ordA = taxoWriter.addCategory(new CategoryPath("a"));
     taxoWriter.close();
     
     Directory dir = newDirectory();
     taxoWriter = new DirectoryTaxonomyWriter(dir);
-    int ordinal = taxoWriter.addCategory(new CategoryPath("b"));
+    int ordB = taxoWriter.addCategory(new CategoryPath("b"));
     taxoWriter.addCategory(new CategoryPath("c"));
     taxoWriter.commit();
     
@@ -330,11 +331,16 @@ public class TestDirectoryTaxonomyWriter extends LuceneTestCase {
     // replace the taxonomy with the input one
     taxoWriter.replaceTaxonomy(input);
     
+    // LUCENE-4633: make sure that category "a" is not added again in any case
+    taxoWriter.addTaxonomy(input, new MemoryOrdinalMap());
+    assertEquals("no categories should have been added", 2, taxoWriter.getSize()); // root + 'a'
+    assertEquals("category 'a' received new ordinal?", ordA, taxoWriter.addCategory(new CategoryPath("a")));
+
     // add the same category again -- it should not receive the same ordinal !
-    int newOrdinal = taxoWriter.addCategory(new CategoryPath("b"));
-    assertNotSame("new ordinal cannot be the original ordinal", ordinal, newOrdinal);
-    assertEquals("ordinal should have been 2 since only one category was added by replaceTaxonomy", 2, newOrdinal);
-    
+    int newOrdB = taxoWriter.addCategory(new CategoryPath("b"));
+    assertNotSame("new ordinal cannot be the original ordinal", ordB, newOrdB);
+    assertEquals("ordinal should have been 2 since only one category was added by replaceTaxonomy", 2, newOrdB);
+
     taxoWriter.close();
     
     long newEpoch = getEpoch(dir);