diff --git a/user/super/com/google/gwt/emul/java/util/stream/Collectors.java b/user/super/com/google/gwt/emul/java/util/stream/Collectors.java
index 3966af525dd..6bf87e56980 100644
--- a/user/super/com/google/gwt/emul/java/util/stream/Collectors.java
+++ b/user/super/com/google/gwt/emul/java/util/stream/Collectors.java
@@ -376,12 +376,11 @@ public static <T> Collector<T,?,List<T>> toList() {
   public static <T, K, U> Collector<T, ?, Map<K, U>> toMap(
       final Function<? super T, ? extends K> keyMapper,
       final Function<? super T, ? extends U> valueMapper) {
-    return toMap(
-        keyMapper,
-        valueMapper,
-        (m1, m2) -> {
-          throw new IllegalStateException("Can't assign multiple values to the same key");
-        });
+    return toMap(keyMapper, valueMapper, null, HashMap::new, true);
+  }
+
+  private static RuntimeException getDuplicateKeyException(Object key) {
+    return new IllegalStateException("Duplicate key " + key);
   }
 
   public static <T, K, U> Collector<T, ?, Map<K, U>> toMap(
@@ -419,18 +418,32 @@ private static <T, R> Function<T, R> disallowNulls(Function<T, R> func) {
       final Function<? super T, ? extends U> valueMapper,
       final BinaryOperator<U> mergeFunction,
       final Supplier<M> mapSupplier) {
+    return toMap(keyMapper, valueMapper, mergeFunction, mapSupplier, false);
+  }
+
+  /*
+   * If unique flag is true, mergeFunction can safely be null.
+   */
+  private static <T, K, U, M extends Map<K, U>> Collector<T, ?, M> toMap(
+      final Function<? super T, ? extends K> keyMapper,
+      final Function<? super T, ? extends U> valueMapper,
+      final BinaryOperator<U> mergeFunction,
+      final Supplier<M> mapSupplier, final boolean unique) {
     return Collector.of(
         mapSupplier,
         (map, item) -> {
           K key = keyMapper.apply(item);
-          U newValue = valueMapper.apply(item);
+          U newValue = Objects.requireNonNull(valueMapper.apply(item));
           if (map.containsKey(key)) {
+            if (unique) {
+              throw getDuplicateKeyException(key);
+            }
             map.put(key, mergeFunction.apply(map.get(key), newValue));
           } else {
             map.put(key, newValue);
           }
         },
-        (m1, m2) -> mergeAll(m1, m2, mergeFunction),
+        (m1, m2) -> unique ? mergeAllUnique(m1, m2) : mergeAll(m1, m2, mergeFunction),
         Collector.Characteristics.IDENTITY_FINISH);
   }
 
@@ -467,6 +480,17 @@ private static <K, V, M extends Map<K, V>> M mergeAll(
     return m1;
   }
 
+  private static <K, V, M extends Map<K, V>> M mergeAllUnique(
+      M m1, M m2) {
+    for (Map.Entry<K, V> entry : m2.entrySet()) {
+      if (m1.get(entry.getKey()) != null) {
+        throw getDuplicateKeyException(entry.getKey());
+      }
+      m1.put(entry.getKey(), entry.getValue());
+    }
+    return m1;
+  }
+
   private static <T, C extends Collection<T>> C addAll(C collection, Collection<T> items) {
     collection.addAll(items);
     return collection;
diff --git a/user/test/com/google/gwt/emultest/java8/util/stream/CollectorsTest.java b/user/test/com/google/gwt/emultest/java8/util/stream/CollectorsTest.java
index 28fbf0da71c..d93eba82ac5 100644
--- a/user/test/com/google/gwt/emultest/java8/util/stream/CollectorsTest.java
+++ b/user/test/com/google/gwt/emultest/java8/util/stream/CollectorsTest.java
@@ -49,6 +49,7 @@
 import java.util.IntSummaryStatistics;
 import java.util.LinkedHashMap;
 import java.util.List;
+import java.util.Locale;
 import java.util.LongSummaryStatistics;
 import java.util.Map;
 import java.util.Optional;
@@ -406,12 +407,13 @@ public void testList() {
   }
 
   public void testMap() {
-    Collector<String, ?, Map<String, String>> c = toMap(Function.identity(), Function.identity());
+    Function<String, String> toUpper = s -> s.toUpperCase(Locale.ROOT);
+    Collector<String, ?, Map<String, String>> c = toMap(Function.identity(), toUpper);
 
     // two distinct items
     Map<String, String> map = new HashMap<>();
-    map.put("a", "a");
-    map.put("b", "b");
+    map.put("a", "A");
+    map.put("b", "B");
     applyItems(map, c, "a", "b");
 
     // inline applyItems and test each to confirm failure for duplicates
@@ -419,15 +421,17 @@ public void testMap() {
       applyItemsWithoutSplitting(c, "a", "a");
       fail("expected IllegalStateException");
     } catch (IllegalStateException expected) {
+      assertEquals("Duplicate key a", expected.getMessage());
     }
     try {
       applyItemsWithSplitting(c, "a", "a");
       fail("expected IllegalStateException");
     } catch (IllegalStateException expected) {
+      assertEquals("Duplicate key a", expected.getMessage());
     }
 
     assertZeroItemsCollectedAs(Collections.emptyMap(), c);
-    assertSingleItemCollectedAs(Collections.singletonMap("a", "a"), c, "a");
+    assertSingleItemCollectedAs(Collections.singletonMap("a", "A"), c, "a");
 
     List<String> seen = new ArrayList<>();
     c = toMap(Function.identity(), Function.identity(), (s, s2) -> {
@@ -441,6 +445,24 @@ public void testMap() {
     assertEquals(Arrays.asList("first: a", "second: a", "first: a", "second: a"), seen);
   }
 
+  public void testMapInvalid() {
+    Collector<String, ?, Map<String, String>> c = toMap(Function.identity(), ignore -> null);
+
+    // inline applyItems and test each to confirm failure for duplicates
+    try {
+      applyItemsWithoutSplitting(c, "a", "b");
+      fail("expected NullPointerException");
+    } catch (NullPointerException expected) {
+      // expected
+    }
+    try {
+      applyItemsWithSplitting(c, "a", "a");
+      fail("expected NullPointerException");
+    } catch (NullPointerException expected) {
+      // expected
+    }
+  }
+
   public void testSet() {
     Collector<String, ?, Set<String>> c = toSet();