Updating Graph with One-to-One relation

Question

I have a class Ricevuta which holds a collection of VoceRicevuta:

public partial class Ricevuta : GestPreBaseBusinessObject
{
    [Key, DatabaseGenerated(DatabaseGeneratedOption.None)]
    public long Id { get; set; }
    ...
    [InverseProperty("Ricevuta")]
    public virtual ObservableListSource<VoceRicevuta> Voci { get; set; }
}

A VoceRicevuta holds optional references to different classes, let's consider only the one to Prestazione class:

public partial class VoceRicevuta
{
    [Key, DatabaseGenerated(DatabaseGeneratedOption.None)]
    public long Id { get; set; }
    public long IdRicevuta { get; set; }
    ....
    public virtual Prestazione Prestazione { get; set; }

    [ForeignKey("IdRicevuta")]
    public virtual Ricevuta Ricevuta { get; set; }
}

Ok, the problem now is the following: I create an Instance of Prestazione (let's call it PreInst) and save it to the database. I detach PreInst from that DbContext instance. Then, I pass PreInst to another form (another DbContext - I must do that), create a Ricevuta, a VoceRicevuta (added to the just created Ricevuta) and assign PreInst to VoceRicevuta Prestazione property. I also made some changes to PreInst. Now I want to save to database the new Ricevuta, the new VoceRicevuta, its relation to PreInst and the changes to PreInst. I run the following:

db.UpdateGraph(Ricevuta, map =>
                map.OwnedCollection(ric => ric.Voci,
                        with => with
                            .OwnedEntity(voce => voce.Prestazione)
                            ));

But i get the error:

"Violation of PRIMARY KEY constraint 'PK_dbo.Prestazioni'. Cannot insert duplicate key in object 'dbo.Prestazioni'. Vlue of duplicate key: (12115)"

I can't understand why! Isn't Graph diff purpose cheching if any part of the graph is already present in Db and behave consquently?

I tried to attach PreInst to DbContext before running the code above and saving. Now the error thrown is:

Multiplicity constraint violated. The role 'Prestazione_VoceRicevuta_Source' of the relationship 'Gestione_Prestazioni.Prestazione_VoceRicevuta' has multiplicity 1 or 0..1.

Any hint?