This will work but they can still cut and paste something in but you'll need to handle that seperate if its an issue.
VB Code:
  1. Private Sub TextBox1_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles TextBox1.KeyPress
  2.         If TextBox1.Text.Length > 0 Then 'must have text
  3.             Dim c() As Char = TextBox1.Text.ToCharArray() 'get char array
  4.             'check last element of char against current keychar
  5.             If e.KeyChar = c(c.GetUpperBound(0)) Then e.Handled = True
  6.         End If
  7.     End Sub