Add testGetDuplicateHeaderMode()

Author: garydgregory

src/test/java/org/apache/commons/csv/CSVFormatTest.java

@@ -692,6 +692,16 @@ public void testGetAllowDuplicateHeaderNames() {
         assertFalse(builder.setDuplicateHeaderMode(DuplicateHeaderMode.DISALLOW).build().getAllowDuplicateHeaderNames());
     }
 
+    @Test
+    public void testGetDuplicateHeaderMode() {
+        final Builder builder = CSVFormat.DEFAULT.builder();
+
+        assertEquals(DuplicateHeaderMode.ALLOW_ALL, builder.build().getDuplicateHeaderMode());
+        assertEquals(DuplicateHeaderMode.ALLOW_ALL, builder.setDuplicateHeaderMode(DuplicateHeaderMode.ALLOW_ALL).build().getDuplicateHeaderMode());
+        assertEquals(DuplicateHeaderMode.ALLOW_EMPTY, builder.setDuplicateHeaderMode(DuplicateHeaderMode.ALLOW_EMPTY).build().getDuplicateHeaderMode());
+        assertEquals(DuplicateHeaderMode.DISALLOW, builder.setDuplicateHeaderMode(DuplicateHeaderMode.DISALLOW).build().getDuplicateHeaderMode());
+    }
+
     @Test
     public void testGetHeader() {
         final String[] header = { "one", "two", "three" };