Hello guys, in my database there are 3 tables

CustomerType
CusID

EventType
EventTypeID

CustomerEventType
CusID
EventTypeID

Dim db = new CustomerEventDataContext
Dim newEvent = new EventType
newEvent.EventTypeID = txtEventID.text
db.EventType.InsertOnSubmit(newEvent)
db.SubmitChanges()

'To select the last ID of event'
Dim lastEventID = (from e in db.EventType Select e.EventTypeID Order By EventTypeID Descending).first()

Dim chkbx As CheckBoxList = CType(form1.FindControl("CheckBoxList1"), CheckBoxList)
Dim newCustomerEventType = New CustomerEventType
Dim i As Integer
For i = 0 To chkbx.Items.Count - 1 Step i + 1
    If (chkbx.Items(i).Selected) Then
        newCustomerEventType.INTEVENTTYPEID = lastEventID
        newCustomerEventType.INTSTUDENTTYPEID = chkbxStudentType.Items(i).Value
        db.CustomerEventType.InsertOnSubmit(newCustomerEventType)
        db.SubmitChanges()
    End If
Next

It works fine when I checked only 1 Single ID of CustomerEventType from CheckBoxList1. It inserts data into EventType with ID 1 and CustomerEventType ID 1. However, when I checked both of them, the error message said

Cannot add an entity that already exists.

Any suggestions please? Thx in advance.