EF Master Detail error msg

Question

hi .. i have master/dtail relation ith EF

 my code :

EngagDBEntities db;


    public EDITENGFrm(object obj)
    {
        InitializeComponent();
        db = new EngagDBEntities();
        if (obj == null)
        {
            engagementBindingSource.DataSource = new Engagement();
            db.Engagements.Add(engagementBindingSource.Current as Engagement);



        }
        else
        {
           engagementBindingSource.DataSource = obj;
           db.Engagements.Attach(engagementBindingSource.Current as Engagement);


        }

    }

 why i have this error when updatig master table :

An entity object cannot be referenced by multiple instances of IEntityChangeTracker