Dim lngItem As Long
Const strSep = ", " 'delimiter

Private Sub UserForm_Initialize()
With ListBox1
    .RowSource = "=Sheet1!A2:A12"
    For lngItem = 0 To ListBox1.ListCount - 1
        .Selected(lngItem) = InStr(1, strSep & ActiveCell.Value & strSep, strSep & .List(lngItem, 0) & strSep)
    Next lngItem
End With
End Sub

Private Sub CommandButton1_Click()
Dim strItems As String
With ListBox1
    For lngItem = 0 To ListBox1.ListCount - 1
        If ListBox1.Selected(lngItem) Then
            strItems = strItems & strSep & ListBox1.List(lngItem, 0)
        End If
    Next lngItem
End With
With ActiveCell
    .NumberFormat = "@"
    .Value = Replace(strItems, strSep, "", 1, 1)
End With
Unload Me
End Sub

Private Sub Worksheet_BeforeDoubleClick(ByVal Target As Range, Cancel As Boolean)
If Target.Column <> 3 Or Target.Row = 1 Then Exit Sub
UserForm1.Show
Cancel = True
End Sub