Web form error message

I am working on a C# asp.net 2010 web form web page I just wrote. Right now I am getting the error

'Cannot add an entity that already exists'. I get that error on the following line of code: attPlanDataContext.SubmitChanges();

I am trying to write to a table that has one to many rows that relate to the main table. When I try to add the second row to the table, that is when the error occurs.

I am hoping you can suggest what I need to change to fix this error. The following is the code that is affected by this error:

protected void submitbutton(object sender, EventArgs e)
{

if (!Page.IsPostBack)
return;

CustomerDataContext attDataContext = new CustomerDataContext();
Customer att = null;
att = new Customer();
InsertCustomer(att);
attDataContext.Customer.InsertOnSubmit(att);
attDataContext.SubmitChanges();
CustomerDataContext attPlanDataContext = new CustomerDataContext();
Plan attplan = null;
attplan = new Plan();
InsertPlans(attplan, att, attPlanDataContext);

}
protected void InsertCustomer(Customer att)
{
// fills in data here
}
protected void InsertPlans(Plan attplan,Customer att, CustomerDataContext attPlanDataContext)
{


for (int i = 0; i < ChkBoxLstPlan.Items.Count; i++)
{

if (ChkBoxLstPlan.Items[i].Selected)
{
attplan.Att_id = att.Att_id;
attplan.Number = ChkBoxLstPlan.Items[i].Value.Substring(0, 5);
// Update the database
attPlanDataContext.Plans.InsertOnSubmit(attplan);
attPlanDataContext.SubmitChanges();
}
}

}