Error:

“a different object with the same identifier value was already associated with the session: 14, of entity: Developer.RoadMap.To.Success.Entities.Customer”

I had something like this in the SaveCustomer method:

if ( customer.CustomerID > 0 )
{
    Session.Update( customer);
}
else
{
    Session.SaveOrUpdate( customer);
}
transaction.Commit();

And this don’t work correctly. Simple change to

if ( customer.CustomerID > 0 )
{
    customer = Session.Merge( customer );
}
else
{
    Session.SaveOrUpdate( customer);
}
transaction.Commit();

resolved my issue.