HDFS-9929. Duplicate keys in NAMENODE_SPECIFIC_KEYS (Contributed by Akira Ajisaka)

(cherry picked from commit f0642b02996ae90d85499ddef2e20bdf88d98c7b)

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/NameNode.java

@@ -268,7 +268,6 @@ public class NameNode extends ReconfigurableBase implements
     DFS_NAMENODE_KERBEROS_INTERNAL_SPNEGO_PRINCIPAL_KEY,
     DFS_HA_FENCE_METHODS_KEY,
     DFS_HA_ZKFC_PORT_KEY,
-    DFS_HA_FENCE_METHODS_KEY
   };
   
   /**

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/TestNameNodeConfiguration.java

@@ -0,0 +1,40 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.hadoop.hdfs.server.namenode;
+
+import org.junit.Test;
+
+import java.util.HashSet;
+import java.util.Set;
+
+import static org.junit.Assert.assertTrue;
+
+public class TestNameNodeConfiguration {
+
+  /**
+   * Detect duplicate keys in {@link NameNode#NAMENODE_SPECIFIC_KEYS}.
+   */
+  @Test
+  public void testNameNodeSpecificKeys() {
+    Set<String> keySet = new HashSet<>();
+    for (String key : NameNode.NAMENODE_SPECIFIC_KEYS) {
+      assertTrue("Duplicate key: " + key
+          + " in NameNode.NAMENODE_SPECIFIC_KEYS.", keySet.add(key));
+    }
+  }
+}