You can modify this:

Dim sDummy As String
Dim bFound As Boolean
On Error Resume Next
'If the text is in the collection, Err = 0, otherwise it's not
sDummy = MyCollection.Item(Text1.Text)
bFound = (Err = 0)
If bFound Then
Err.Clear
MsgBox "Already in list"
Else
List1.AddItem Text1.Text
MyCollection.Add Text1.Text, Text1.Text
End If