Added unit test.

src/test/java/org/apache/commons/math4/util/MathArraysTest.java

@@ -1106,6 +1106,23 @@ public class MathArraysTest {
         }
     }
 
+    @Test
+    public void testShuffleNoDuplicates() {
+        final int n = 100;
+        final int[] orig = MathArrays.natural(n);
+        MathArrays.shuffle(orig);
+
+        // Test that all (unique) entries exist in the shuffled array.
+        final int[] count = new int[n];
+        for (int i = 0; i < n; i++) {
+            count[orig[i]] += 1;
+        }
+
+        for (int i = 0; i < n; i++) {
+            Assert.assertEquals(1, count[i]);
+        }
+    }
+
     @Test
     public void testShuffleTail() {
         final int[] orig = new int[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };