You could check if the ordlines contains the values you want to add
If OrdLines.Contains(555) Or OrdLines.Contains("This is the fifth item") Then
MessageBox.Show("One of these items allready excists")
Else
OrdLines.Add(555, "This is the fifth item")
End If
But when the value you want to add is 5555 then you would have a messagebox
showing that the value already excists.
To avoid that you could use an equals and check every value in your list
For Each item In OrdLines
If item.Equals(555) Then
MessageBox.Show("555")
ElseIf Not item.Equals(555) Then
OrdLines.Add(555, "This is the fifth item")
End If
Next