[LANG-1795] Fix EnumUtils.getEnumMap throw IllegalStateException when keyFunction produces duplicate keys

src/main/java/org/apache/commons/lang3/EnumUtils.java

@@ -302,7 +302,7 @@ public static <E extends Enum<E>> Map<String, E> getEnumMap(final Class<E> enumC
      * @since 3.13.0
      */
     public static <E extends Enum<E>, K> Map<K, E> getEnumMap(final Class<E> enumClass, final Function<E, K> keyFunction) {
-        return stream(enumClass).collect(Collectors.toMap(keyFunction::apply, Function.identity()));
+        return stream(enumClass).collect(Collectors.toMap(keyFunction::apply, Function.identity(), (existing, replacement) -> existing));
     }
 
     /**

src/test/java/org/apache/commons/lang3/EnumUtilsTest.java

@@ -621,6 +621,10 @@ void testStream() {
         assertEquals(0, EnumUtils.stream(null).count());
     }
 
+    @Test
+    void testGetEnumMapOnDuplicateKey() {
+        assertEquals(1,EnumUtils.getEnumMap(DuplicateKeyEnum.class, e -> e.key).size());
+    }
 }
 
 enum Month {
@@ -667,3 +671,11 @@ public int getValue() {
         return value;
     }
 }
+
+enum DuplicateKeyEnum {
+    A("x"), B("x");
+
+    final String key;
+
+    DuplicateKeyEnum(String key) { this.key = key; }
+}