# UNDF: UNDF-2026-000000706
--- a/resolver-dns/src/main/java/io/netty/resolver/dns/DnsResolveContext.java
+++ b/resolver-dns/src/main/java/io/netty/resolver/dns/DnsResolveContext.java
@@ -55,6 +55,7 @@ import java.util.List;
 import java.util.Locale;
 import java.util.Map;
 import java.util.NoSuchElementException;
+import java.util.LinkedHashSet;
 import java.util.Set;

@@ -117,6 +118,8 @@ abstract class DnsResolveContext<T> {

     private List<T> finalResult;
+    // Tracks items already in finalResult for O(1) duplicate detection.
+    // Only allocated when isDuplicateAllowed() returns false (address resolution).
+    private Set<T> finalResultSet;
     private int allowedQueries;
     private boolean triedCNAME;

@@ -903,17 +906,15 @@ abstract class DnsResolveContext<T> {
             if (!promise.isDone()) {
-                // We want to ensure we do not have duplicates in finalResult as this may be unexpected.
-                //
-                // While using a LinkedHashSet or HashSet may sound like the perfect fit for this we will use an
-                // ArrayList here as duplicates should be found quite unfrequently in the wild and we dont want to pay
-                // for the extra memory copy and allocations in this cases later on.
                 if (finalResult == null) {
                     finalResult = new ArrayList<T>(8);
+                    if (!isDuplicateAllowed()) {
+                        finalResultSet = new LinkedHashSet<T>(8);
+                    }
                     finalResult.add(converted);
-                } else if (isDuplicateAllowed() || !finalResult.contains(converted)) {
+                    if (finalResultSet != null) {
+                        finalResultSet.add(converted);
+                    }
+                } else if (isDuplicateAllowed() || finalResultSet.add(converted)) {
+                    // finalResultSet.add() returns false if already present: O(1) duplicate check
                     finalResult.add(converted);
                 } else {
                     shouldRelease = true;