diff --git a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/security/UserGroupInformation.java b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/security/UserGroupInformation.java
index c8a67c76cde..6f90fbd17f2 100644
--- a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/security/UserGroupInformation.java
+++ b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/security/UserGroupInformation.java
@@ -35,6 +35,7 @@
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.Iterator;
+import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
@@ -1415,7 +1416,8 @@ private synchronized Credentials getCredentialsInternal() {
   public synchronized String[] getGroupNames() {
     ensureInitialized();
     try {
-      List<String> result = groups.getGroups(getShortUserName());
+      Set<String> result = new LinkedHashSet<String>
+        (groups.getGroups(getShortUserName()));
       return result.toArray(new String[result.size()]);
     } catch (IOException ie) {
       LOG.warn("No groups available for user " + getShortUserName());
diff --git a/hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/security/TestUserGroupInformation.java b/hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/security/TestUserGroupInformation.java
index 442a603a238..31a660e724f 100644
--- a/hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/security/TestUserGroupInformation.java
+++ b/hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/security/TestUserGroupInformation.java
@@ -17,6 +17,7 @@
 package org.apache.hadoop.security;
 
 import org.apache.hadoop.conf.Configuration;
+import org.apache.hadoop.fs.CommonConfigurationKeysPublic;
 import org.apache.hadoop.io.Text;
 import org.apache.hadoop.metrics2.MetricsRecordBuilder;
 import org.apache.hadoop.security.UserGroupInformation.AuthenticationMethod;
@@ -33,8 +34,9 @@
 import java.io.IOException;
 import java.io.InputStreamReader;
 import java.security.PrivilegedExceptionAction;
-import java.util.ArrayList;
 import java.util.Collection;
+import java.util.LinkedHashSet;
+import java.util.Set;
 
 import static org.apache.hadoop.fs.CommonConfigurationKeys.HADOOP_USER_GROUP_METRICS_PERCENTILES_INTERVALS;
 import static org.apache.hadoop.fs.CommonConfigurationKeysPublic.HADOOP_SECURITY_AUTH_TO_LOCAL;
@@ -204,7 +206,7 @@ public void testGetServerSideGroups() throws IOException,
 
     System.out.println(userName + ":" + line);
    
-    ArrayList<String> groups = new ArrayList<String> ();    
+    Set<String> groups = new LinkedHashSet<String> ();    
     String[] tokens = line.split(Shell.TOKEN_SEPARATOR_REGEX);
     for(String s: tokens) {
       groups.add(s);