Why is Entity Framework creating A new Row in my table each time I save changes

I can't see the wood through the trees with this one. I'm pretty sure the answer is staring me in the face but I'm just missing the obvious.

This is my code to create a Customer WebSession. The Customer Web Session object is an EF Code First object (For testing purposes I drop the DB each time I run this and seed it with a bit of sample data). On going to the apply now screen, the user is asked to enter some compulsory details. There are only about 6 required fields in the database including the id and the ApplyNow page covers these.

        'GET: /Home/ApplyNow
        Function ApplyNow() As ActionResult
            Dim customerWebSession = New CKCustomerWebSession
            customerWebSession.GeneratedCode = CodeGenerator.CodeCheckUnique()
            Return View(customerWebSession)
        End Function

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On the post request, I check that the model is valid and if it is then I add my WebSession object to the context and submit the change to the database. I then redirect the user to a new action method on a seperate controller and pass in the Web Session Object.

        'POST: /Home/ApplyNow
        <HttpPost()>
        Function ApplyNow(WebSession As CKCustomerWebSession) As ActionResult
            If ModelState.IsValid Then
                db.WebSessions.Add(WebSession)
                db.SaveChanges()
                Return RedirectToAction("Page1", "Session", WebSession)
            Else
                Return View(WebSession)
            End If
        End Function

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Here I simply GET a view and pass in my existing Web Session Object. This view allows the user to enter more details and submit them.

        'GET: /Session/Page1
        Function Page1(WebSession As CKCustomerWebSession) As ActionResult
            Return View(WebSession)
        End Function

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Now this is where the problem seems to kick in, on the POST back of the new user data. Instead of adding it to the existing WebSession object it creates a brand new one and adds a new row into my database. I want it to update the existing row instead.

        'POST: /Session/Page1
        <HttpPost()>
        <ActionName("Page1")>
        Function Page1Post(WebSession As CKCustomerWebSession) As ActionResult
            If ModelState.IsValid Then
                    db.WebSessions.Add(WebSession)
                    db.SaveChanges()
                Return RedirectToAction("Page2", "Session", WebSession)
            Else
                Return View(WebSession)
            End If
        End Function

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Any help that can be provided on this would be much appreciated.

Thanks