Public Shared Function WithDuplicateHandling( _
   ByVal items As IEnumerable(Of T), _
   ByVal comparer As IComparer(Of T), _
   ByVal duplicateHandling As DuplicateHandling _
) As IEnumerable(Of T)

public static IEnumerable<T> WithDuplicateHandling( 
   IEnumerable<T> items,
   IComparer<T> comparer,
   DuplicateHandling duplicateHandling
)

char[] x = "aeiou".ToCharArray();
char[] y = "bcdfg".ToCharArray();
char[] z = "ez".ToCharArray();

var order = OrderedEnumeration<char>.Merge(x, y, z);
Assert.AreEqual("abcdeefgiouz", new string(new List<char>(order).ToArray()));

order = OrderedEnumeration<char>.Merge(Comparer<char>.Default, DuplicateHandling.LastValueWins, x, y, z);
Assert.AreEqual("abcdefgiouz", new string(new List<char>(order).ToArray()));

order = OrderedEnumeration<char>.Merge(Comparer<char>.Default, x, y);
order = OrderedEnumeration<char>.WithDuplicateHandling(order, Comparer<char>.Default,
                                                       DuplicateHandling.FirstValueWins);
Assert.AreEqual("abcdefgiou", new string(new List<char>(order).ToArray()));

Dim x As Char() = "aeiou".ToCharArray()
Dim y As Char() = "bcdfg".ToCharArray()
Dim z As Char() = "ez".ToCharArray()

Dim order As var = OrderedEnumeration(Of Char).Merge(x, y, z)
Assert.AreEqual("abcdeefgiouz", New String(New List(Of Char)(order).ToArray()))

order = OrderedEnumeration(Of Char).Merge(Comparer(Of Char).[Default], DuplicateHandling.LastValueWins, x, y, z)
Assert.AreEqual("abcdefgiouz", New String(New List(Of Char)(order).ToArray()))

order = OrderedEnumeration(Of Char).Merge(Comparer(Of Char).[Default], x, y)
order = OrderedEnumeration(Of Char).WithDuplicateHandling(order, Comparer(Of Char).[Default], DuplicateHandling.FirstValueWins)
Assert.AreEqual("abcdefgiou", New String(New List(Of Char)(order).ToArray()))