Listbox is adding too many entries - visual-studio-2013

Question

What I want is for this program to do is to show Microsoft Outlook 2010 in "installed" listbox if it's installed and "notinstalled" if it's not installed. "listbox1" has a list of all installed applications in it on form load.
The issue is that while it does work for the "installed" portion, it lists the application many times in the "notinstalled" box. I only want it to show up once.
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Dim regkey, subkey As Microsoft.Win32.RegistryKey
Dim value As String
Dim regpath As String = "Software\Microsoft\Windows\CurrentVersion\Uninstall"
regkey = My.Computer.Registry.LocalMachine.OpenSubKey(regpath)
Dim subkeys() As String = regkey.GetSubKeyNames
Dim includes As Boolean
For Each subk As String In subkeys
subkey = regkey.OpenSubKey(subk)
value = subkey.GetValue("DisplayName", "")
If value <> "" Then
includes = True
If value.IndexOf("Hotfix") <> -1 Then includes = False
If value.IndexOf("Security Update") <> -1 Then includes = False
If value.IndexOf("Update for") <> -1 Then includes = False
If includes = True Then ListBox1.Items.Add(value)
End If
Next
Dim count As Integer = (ListBox1.Items.Count - 1)
Dim words As String
Dim softName As String
softName = "Microsoft Outlook 2010"
For a = 0 To count
words = ListBox1.Items.Item(a)
If InStr(words.ToLower, softName.ToLower) Then
Installed.Items.Add(words)
Else
NotInstalled.Items.Add(softName)
End If
Next

Answer 1

I think you should try something like. (I'm a c# guy, so I hope my syntax is right.)
Dim isOutlookInstalled = False
softName = "Microsoft Outlook 2010"
For a = 0 To count
words = ListBox1.Items.Item(a)
If InStr(words.ToLower, softName.ToLower) Then
Installed.Items.Add(words)
End If
Next
If isOutlookInstalled <> True
NotInstalled.Items.Add(softName)
End If

Answer 2

The repeating string is due to a simple error due to the call to NotInstalled.Items.Add(softName) for each item inspected. You probably want to add it only at the end of the loop.
However you could simplify your code with a bit of Linq
Dim result = words.Where(Function(x) x.ToLower().IndexOf("microsoft outlook 2010") >= 0)
if result IsNot Nothing then
Installed.Items.AddRange(result.ToArray)
else
NotInstalled.Items.Add(softName)
end if
But you should consider some problems in your code. If Outlook is installed as part of Office there is not an entry for it in the uninstall section. Then there is the problem of the automatic redirection to different section of the registry if you are running on a 64bit system. And what if your Outlook is a 32bit version installed on a 64bit system and your app run in AnyCPU mode? It is not a trivial task to account for all these possibility. Just to warn you