# UNDF: UNDF-2026-000000343
--- a/zookeeper-server/src/main/java/org/apache/zookeeper/server/PrepRequestProcessor.java
+++ b/zookeeper-server/src/main/java/org/apache/zookeeper/server/PrepRequestProcessor.java
@@ -943,12 +943,14 @@
     private static List<ACL> removeDuplicates(final List<ACL> acls) {
         if (acls == null || acls.isEmpty()) {
             return Collections.emptyList();
         }
-        // This would be done better with a Set but ACL hashcode/equals do not
-        // allow for null values
-        final ArrayList<ACL> retval = new ArrayList<>(acls.size());
+        // LinkedHashSet gives O(1) contains/add while preserving insertion order.
+        // ACL.hashCode() and ACL.equals() are defined on (perms, id) — null Id fields
+        // are handled by the Thrift-generated equals(); no special-casing is needed.
+        final LinkedHashSet<ACL> seen = new LinkedHashSet<>(acls.size() * 2);
         for (final ACL acl : acls) {
-            if (!retval.contains(acl)) {
-                retval.add(acl);
-            }
+            seen.add(acl);
         }
-        return retval;
+        return new ArrayList<>(seen);
     }