Retrieving Multiple Instances Of The Same Session Variable

I'm having problems with retrieving multiple instances of a session variable from an InProc session state. In the following code I persist a simple BusinessObject into a session variable on the Page_Load event. On the click of a button I try to retrieve the object back into 2 new declared instances of the same BusinessObject. All works great until I change one of the properties in the first instance, it changes the second instance as well. Is this normal behaviour? I would have thought as these were new instances they wouldn't demonstrate static behaviour?

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
If Not Page.IsPostBack Then
' create a new instance of a business object and set a containg variable
Dim BO As New BusinessObject
BO.SomeVariable = "test"
' persist to inproc session
Session("BO") = BO
End If
End Sub

Protected Sub btnRetrieveSessionVariable_Click(ByVal sender As Object, ByVal e As EventArgs) Handles btnRetrieveSessionVariable.Click
' retrieve the session variable to a new instance of BusinessObject
Dim BO1 As New BusinessObject
If Not Session("BO") Is Nothing Then BO1 = Session("BO")
' retrieve the session variable to a new instance of BusinessObject...............................