diff --git a/gson/src/main/java/com/google/gson/internal/bind/MapTypeAdapterFactory.java b/gson/src/main/java/com/google/gson/internal/bind/MapTypeAdapterFactory.java
index a72bb48cc1..b285665338 100644
--- a/gson/src/main/java/com/google/gson/internal/bind/MapTypeAdapterFactory.java
+++ b/gson/src/main/java/com/google/gson/internal/bind/MapTypeAdapterFactory.java
@@ -34,8 +34,10 @@
 import java.io.IOException;
 import java.lang.reflect.Type;
 import java.util.ArrayList;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
+import java.util.Set;
 
 /**
  * Adapts maps to either JSON objects or JSON arrays.
@@ -220,8 +222,15 @@ public void write(JsonWriter out, Map<K, V> map) throws IOException {
 
       if (!complexMapKeySerialization) {
         out.beginObject();
+        Set<String> emittedNames = new HashSet<>();
         for (Map.Entry<K, V> entry : map.entrySet()) {
-          out.name(String.valueOf(entry.getKey()));
+          String name = String.valueOf(entry.getKey());
+          // Mirror the read path's duplicate-key rejection: distinct keys whose string form
+          // collides would otherwise silently emit a duplicate JSON object member name.
+          if (!emittedNames.add(name)) {
+            throw new JsonSyntaxException("duplicate key: " + name);
+          }
+          out.name(name);
           valueTypeAdapter.write(out, entry.getValue());
         }
         out.endObject();
@@ -250,9 +259,14 @@ public void write(JsonWriter out, Map<K, V> map) throws IOException {
         out.endArray();
       } else {
         out.beginObject();
+        Set<String> emittedNames = new HashSet<>();
         for (int i = 0, size = keys.size(); i < size; i++) {
           JsonElement keyElement = keys.get(i);
-          out.name(keyToString(keyElement));
+          String name = keyToString(keyElement);
+          if (!emittedNames.add(name)) {
+            throw new JsonSyntaxException("duplicate key: " + name);
+          }
+          out.name(name);
           valueTypeAdapter.write(out, values.get(i));
         }
         out.endObject();
diff --git a/gson/src/test/java/com/google/gson/regression/MapTypeAdapterDuplicateKeyTest.java b/gson/src/test/java/com/google/gson/regression/MapTypeAdapterDuplicateKeyTest.java
new file mode 100644
index 0000000000..5a83fc75e5
--- /dev/null
+++ b/gson/src/test/java/com/google/gson/regression/MapTypeAdapterDuplicateKeyTest.java
@@ -0,0 +1,58 @@
+// Generated by Quality Playbook v1.5.8 — https://github.com/andrewstellman/quality-playbook
+// Author: Andrew Stellman · Date: 2026-06-04 · Project: gson
+package com.google.gson.regression;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import com.google.gson.Gson;
+import com.google.gson.JsonSyntaxException;
+import com.google.gson.reflect.TypeToken;
+import java.util.LinkedHashMap;
+import java.util.Map;
+import org.junit.Test;
+
+/** Regression test: map serialization must not silently emit duplicate JSON member names. */
+public class MapTypeAdapterDuplicateKeyTest {
+  @Test
+  public void mapWriteDoesNotEmitDuplicateNames() {
+    Gson gson = new Gson();
+    Object k1 =
+        new Object() {
+          @Override
+          public String toString() {
+            return "k";
+          }
+        };
+    Object k2 =
+        new Object() {
+          @Override
+          public String toString() {
+            return "k";
+          }
+        };
+    Map<Object, String> map = new LinkedHashMap<>();
+    map.put(k1, "a");
+    map.put(k2, "b");
+    // Confirm two distinct anonymous-instance keys (defensive against future key-type refactors).
+    assertThat(map).hasSize(2);
+
+    String json;
+    try {
+      json = gson.toJson(map, new TypeToken<Map<Object, String>>() {}.getType());
+    } catch (JsonSyntaxException expected) {
+      // Acceptable fix: throw JsonSyntaxException naming the duplicate key. Narrow match —
+      // message must reference "duplicate key:" followed by the literal colliding name "k",
+      // not merely contain the word "key" from any unrelated JSON syntax error.
+      assertThat(expected).hasMessageThat().containsMatch("duplicate key:\\s*k(?:\\W|$)");
+      return;
+    }
+
+    // Otherwise: gson emitted JSON without throwing. Pin the object-form serialization (not the
+    // complex-key array-form) so the indexOf "k": substring check is exercising the bug's locus.
+    assertThat(json).startsWith("{");
+    int first = json.indexOf("\"k\":");
+    assertThat(first).isAtLeast(0);
+    int second = json.indexOf("\"k\":", first + 1);
+    assertThat(second).isEqualTo(-1);
+  }
+}