Small Trial

  Dim strList As New List(Of String)
    Dim break As Integer = 0
    Private Sub Button3_Click(sender As Object, e As EventArgs) Handles Button3.Click
        lstOutput.Items.Clear()

        Dim letterToGenerate As Integer = 1
        Dim str As String = "Nic"
        Dim sb As New StringBuilder
        Dim letterLength As Integer = 2
        Dim Ran As New Random
        Dim i As Integer = 0




        Do While i < letterToGenerate
            sb.Clear()

            For j As Integer = 0 To letterLength
                Dim index As String = Ran.Next(0, str.Length)
                sb.Append(str(index))
            Next

            If Not strList.Contains(sb.ToString) Then
                strList.Add(sb.ToString)
                txtOutput.Text = sb.ToString
                i += 1
            Else
                lstOutput.Items.Add("Repeated Letter - " & sb.ToString & " ,Skip it")
                break += 1
            End If

            If break > 200 Then ' your array length * math formula to get the max combination
                lstOutput.Items.Add("Exit loop")
                Exit Do
            End If
        Loop

        lstOutput.Items.Add("Full collection is : ")
        For Each letter In strList
            lstOutput.Items.Add(letter)
            lstOutput.SelectedIndex = lstOutput.Items.Count - 1
        Next
    End Sub