CSV-264: Fixes requested by contributor.

Added back public API methods.
Fixed since version on new methods added by this PR.
Updated check-supressions to have new line number.
Reverted pre-existing tests to use public API methods that were added back.

Author: SethFalco

src/main/java/org/apache/commons/csv/CSVFormat.java

@@ -884,11 +884,21 @@ public String format(final Object... values) {
         }
     }
 
+    /**
+     * Returns true if and only if duplicate names are allowed in the headers.
+     *
+     * @return whether duplicate header names are allowed
+     * @since 1.7
+     */
+    public boolean getAllowDuplicateHeaderNames() {
+        return duplicateHeaderMode == DuplicateHeaderMode.ALLOW_ALL;
+    }
+
     /**
      * Returns how duplicate headers are handled.
      *
      * @return if duplicate header values are allowed, allowed conditionally, or disallowed.
-     * @since 1.7
+     * @since 1.9
      */
     public DuplicateHeaderMode getDuplicateHeaderMode() {
         return duplicateHeaderMode;
@@ -1686,13 +1696,30 @@ public CSVFormat withAllowDuplicateHeaderNames() {
         return withDuplicateHeaderMode(DuplicateHeaderMode.ALLOW_ALL);
     }
 
+    /**
+     * Returns a new {@code CSVFormat} with duplicate header names behavior set to the given value.
+     *
+     * @param allowDuplicateHeaderNames the duplicate header names behavior, true to allow, false to disallow.
+     * @return a new {@code CSVFormat} with duplicate header names behavior set to the given value.
+     * @since 1.7
+     */
+    public CSVFormat withAllowDuplicateHeaderNames(final boolean allowDuplicateHeaderNames) {
+        final DuplicateHeaderMode mode =
+            (allowDuplicateHeaderNames) ? DuplicateHeaderMode.ALLOW_ALL : DuplicateHeaderMode.ALLOW_EMPTY;
+
+        return new CSVFormat(delimiter, quoteCharacter, quoteMode, commentMarker, escapeCharacter,
+                ignoreSurroundingSpaces, ignoreEmptyLines, recordSeparator, nullString, headerComments, header,
+                skipHeaderRecord, allowMissingColumnNames, ignoreHeaderCase, trim, trailingDelimiter, autoFlush,
+                mode);
+    }
+
     /**
      * Returns a new {@code CSVFormat} with duplicate header names behavior set to the given value.
      *
      * @param duplicateHeaderMode the duplicate header names behavior, to allow, allow conditionally,
      * or disable duplicates.
      * @return a new {@code CSVFormat} with duplicate header names behavior set to the given value.
-     * @since 1.7
+     * @since 1.9
      */
     public CSVFormat withDuplicateHeaderMode(final DuplicateHeaderMode duplicateHeaderMode) {
         return new CSVFormat(delimiter, quoteCharacter, quoteMode, commentMarker, escapeCharacter,

src/main/java/org/apache/commons/csv/CSVParser.java

@@ -505,9 +505,11 @@ private Headers createHeaders() throws IOException {
                     }
 
                     final boolean containsHeader = header != null && hdrMap.containsKey(header);
-                    final DuplicateHeaderMode headerRule = this.format.getDuplicateHeaderMode();
+                    final DuplicateHeaderMode headerMode = this.format.getDuplicateHeaderMode();
+                    final boolean duplicatesAllowed = headerMode == DuplicateHeaderMode.ALLOW_ALL;
+                    final boolean emptyDuplicatesAllowed = headerMode == DuplicateHeaderMode.ALLOW_EMPTY;
 
-                    if (containsHeader && headerRule != DuplicateHeaderMode.ALLOW_ALL && !(emptyHeader && headerRule == DuplicateHeaderMode.ALLOW_EMPTY)) {
+                    if (containsHeader && !duplicatesAllowed && !(emptyHeader && emptyDuplicatesAllowed)) {
                         throw new IllegalArgumentException(
                             String.format(
                                 "The header contains a duplicate name: \"%s\" in %s. If this is valid then use CSVFormat.withDuplicateHeaderMode().",

src/site/resources/checkstyle/checkstyle-suppressions.xml

@@ -19,5 +19,5 @@
     "-//Checkstyle//DTD SuppressionFilter Configuration 1.2//EN"
     "https://checkstyle.org/dtds/suppressions_1_2.dtd">
 <suppressions>
-  <suppress checks="LineLength" files="[\\/]CSVParser\.java$" lines="511"/>
+  <suppress checks="LineLength" files="[\\/]CSVParser\.java$" lines="515"/>
 </suppressions>

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

@@ -100,11 +100,11 @@ public void testDuplicateHeaderElements() {
     public void testDuplicateHeaderElementsFalse() {
         assertThrows(
                 IllegalArgumentException.class,
-                () -> CSVFormat.DEFAULT.withDuplicateHeaderMode(DuplicateHeaderMode.DISALLOW).withHeader("A", "A"));
+                () -> CSVFormat.DEFAULT.withAllowDuplicateHeaderNames(false).withHeader("A", "A"));
     }
 
     public void testDuplicateHeaderElementsTrue() {
-        CSVFormat.DEFAULT.withDuplicateHeaderMode(DuplicateHeaderMode.ALLOW_ALL).withHeader("A", "A");
+        CSVFormat.DEFAULT.withAllowDuplicateHeaderNames(true).withHeader("A", "A");
     }
 
     @Test

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

@@ -298,7 +298,7 @@ public void testDefaultFormat() throws IOException {
     @Test
     public void testDuplicateHeadersNotAllowed() {
         assertThrows(IllegalArgumentException.class, () -> CSVParser.parse("a,b,a\n1,2,3\nx,y,z",
-            CSVFormat.DEFAULT.withHeader().withDuplicateHeaderMode(DuplicateHeaderMode.DISALLOW)));
+            CSVFormat.DEFAULT.withHeader().withAllowDuplicateHeaderNames(false)));
     }
 
     @Test