Index: java/org/apache/catalina/util/Extension.java
===================================================================
--- java/org/apache/catalina/util/Extension.java	(revision 1740624)
+++ java/org/apache/catalina/util/Extension.java	(working copy)
@@ -19,6 +19,7 @@
 package org.apache.catalina.util;
 
 
+import java.util.Objects;
 import java.util.StringTokenizer;
 
 
@@ -248,7 +249,27 @@
 
     }
 
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (!(o instanceof Extension)) return false;
+        Extension extension = (Extension) o;
+        return Objects.equals(extensionName, extension.extensionName) &&
+                Objects.equals(implementationURL, extension.implementationURL) &&
+                Objects.equals(implementationVendor, extension.implementationVendor) &&
+                Objects.equals(implementationVendorId, extension.implementationVendorId) &&
+                Objects.equals(implementationVersion, extension.implementationVersion) &&
+                Objects.equals(specificationVendor, extension.specificationVendor) &&
+                Objects.equals(specificationVersion, extension.specificationVersion);
+    }
 
+    @Override
+    public int hashCode() {
+        return Objects.hash(extensionName, implementationURL, implementationVendor, implementationVendorId,
+                implementationVersion, specificationVendor, specificationVersion);
+    }
+
+
     // -------------------------------------------------------- Private Methods
 
 
Index: java/org/apache/catalina/util/ExtensionValidator.java
===================================================================
--- java/org/apache/catalina/util/ExtensionValidator.java	(revision 1740624)
+++ java/org/apache/catalina/util/ExtensionValidator.java	(working copy)
@@ -21,8 +21,10 @@
 import java.io.IOException;
 import java.io.InputStream;
 import java.util.ArrayList;
+import java.util.HashSet;
 import java.util.Iterator;
 import java.util.Locale;
+import java.util.Set;
 import java.util.StringTokenizer;
 import java.util.jar.JarInputStream;
 import java.util.jar.Manifest;
@@ -56,10 +58,10 @@
     private static final StringManager sm =
             StringManager.getManager("org.apache.catalina.util");
 
-    private static volatile ArrayList<Extension> containerAvailableExtensions =
+    private static volatile Set<Extension> containerAvailableExtensions =
             null;
-    private static final ArrayList<ManifestResource> containerManifestResources =
-            new ArrayList<>();
+    private static final Set<ManifestResource> containerManifestResources =
+            new HashSet<>();
 
 
     // ----------------------------------------------------- Static Initializer
@@ -129,7 +131,7 @@
                     throws IOException {
 
         String appName = context.getName();
-        ArrayList<ManifestResource> appManifestResources = new ArrayList<>();
+        Set<ManifestResource> appManifestResources = new HashSet<>();
 
         // Web application manifest
         WebResource resource = resources.getResource("/META-INF/MANIFEST.MF");
@@ -206,15 +208,15 @@
      * @return true if manifest resource file requirements are met
      */
     private static boolean validateManifestResources(String appName,
-            ArrayList<ManifestResource> resources) {
+            Set<ManifestResource> resources) {
         boolean passes = true;
         int failureCount = 0;
-        ArrayList<Extension> availableExtensions = null;
+        Set<Extension> availableExtensions = null;
 
         Iterator<ManifestResource> it = resources.iterator();
         while (it.hasNext()) {
             ManifestResource mre = it.next();
-            ArrayList<Extension> requiredList = mre.getRequiredExtensions();
+            Set<Extension> requiredList = mre.getRequiredExtensions();
             if (requiredList == null) {
                 continue;
             }
@@ -299,21 +301,21 @@
     *
     * @return HashMap Map of available extensions
     */
-    private static ArrayList<Extension> buildAvailableExtensionsList(
-            ArrayList<ManifestResource> resources) {
+    private static Set<Extension> buildAvailableExtensionsList(
+            Set<ManifestResource> resources) {
 
-        ArrayList<Extension> availableList = null;
+        Set<Extension> availableList = null;
 
         Iterator<ManifestResource> it = resources.iterator();
         while (it.hasNext()) {
             ManifestResource mre = it.next();
-            ArrayList<Extension> list = mre.getAvailableExtensions();
+            Set<Extension> list = mre.getAvailableExtensions();
             if (list != null) {
                 Iterator<Extension> values = list.iterator();
                 while (values.hasNext()) {
                     Extension ext = values.next();
                     if (availableList == null) {
-                        availableList = new ArrayList<>();
+                        availableList = new HashSet<>();
                         availableList.add(ext);
                     } else {
                         availableList.add(ext);
Index: java/org/apache/catalina/util/ManifestResource.java
===================================================================
--- java/org/apache/catalina/util/ManifestResource.java	(revision 1740624)
+++ java/org/apache/catalina/util/ManifestResource.java	(working copy)
@@ -17,7 +17,10 @@
 package org.apache.catalina.util;
 
 import java.util.ArrayList;
+import java.util.HashSet;
 import java.util.Iterator;
+import java.util.Objects;
+import java.util.Set;
 import java.util.jar.Attributes;
 import java.util.jar.Manifest;
 
@@ -37,8 +40,8 @@
     public static final int WAR = 2;
     public static final int APPLICATION = 3;
 
-    private ArrayList<Extension> availableExtensions = null;
-    private ArrayList<Extension> requiredExtensions = null;
+    private Set<Extension> availableExtensions = null;
+    private Set<Extension> requiredExtensions = null;
 
     private final String resourceName;
     private final int resourceType;
@@ -64,7 +67,7 @@
      *
      * @return List of available extensions
      */
-    public ArrayList<Extension> getAvailableExtensions() {
+    public Set<Extension> getAvailableExtensions() {
         return availableExtensions;
     }
 
@@ -73,7 +76,7 @@
      *
      * @return List of required extensions
      */
-    public ArrayList<Extension> getRequiredExtensions() {
+    public Set<Extension> getRequiredExtensions() {
         return requiredExtensions;
     }
 
@@ -136,7 +139,23 @@
         return (sb.toString());
     }
 
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (!(o instanceof ManifestResource)) return false;
+        ManifestResource that = (ManifestResource) o;
+        return Objects.equals(availableExtensions, that.availableExtensions) &&
+                Objects.equals(requiredExtensions, that.requiredExtensions) &&
+                Objects.equals(resourceName, that.resourceName) &&
+                resourceType == that.resourceType;
+    }
 
+    @Override
+    public int hashCode() {
+        return Objects.hash(availableExtensions, requiredExtensions, resourceName, resourceType);
+    }
+
+
     // -------------------------------------------------------- Private Methods
 
     private void processManifest(Manifest manifest) {
@@ -154,7 +173,7 @@
      * @return List of required extensions, or null if the application
      * does not require any extensions
      */
-    private ArrayList<Extension> getRequiredExtensions(Manifest manifest) {
+    private Set<Extension> getRequiredExtensions(Manifest manifest) {
 
         Attributes attributes = manifest.getMainAttributes();
         String names = attributes.getValue("Extension-List");
@@ -161,7 +180,7 @@
         if (names == null)
             return null;
 
-        ArrayList<Extension> extensionList = new ArrayList<>();
+        Set<Extension> extensionList = new HashSet<>();
         names += " ";
 
         while (true) {
@@ -201,7 +220,7 @@
      * @return List of available extensions, or null if the web application
      * does not bundle any extensions
      */
-    private ArrayList<Extension> getAvailableExtensions(Manifest manifest) {
+    private Set<Extension> getAvailableExtensions(Manifest manifest) {
 
         Attributes attributes = manifest.getMainAttributes();
         String name = attributes.getValue("Extension-Name");
@@ -208,7 +227,7 @@
         if (name == null)
             return null;
 
-        ArrayList<Extension> extensionList = new ArrayList<>();
+        Set<Extension> extensionList = new HashSet<>();
 
         Extension extension = new Extension();
         extension.setExtensionName(name);