C# - Cannot Add An Entity With A Key That Is Already In Use?

I'm having a bit of trouble trying to add an object to the database using LINQ to SQL.

I have three tables in my database:

--------------------------------------------
tblShows
--------------------------------------------
ShowID | Name
--------------------------------------------

--------------------------------------------
tblShowEvents
--------------------------------------------
ShowEventID | ShowID | Name
--------------------------------------------

--------------------------------------------
tblShowEventAttendees
--------------------------------------------
ShowEventAttendeeID | ShowEventID | Name
--------------------------------------------

A Show has many Events, and each Event has many Attendees.

I have a page where I'm creating a Show. On the page, I'm adding Events to the Show, and Attendees to each Event, and then saving the show all at once.

Here is my code:

Show show = new Show();
show.Name = txtName.Text.Trim();

//add ShowEvents to Show
//showEventsList is a variable containing a List of ShowEvents
foreach (ShowEvent ev in showEventsList)
{
show.ShowEvents.Add(new ShowEvent
{
ShowID = show.ShowID,
Name = ev.Name
});
}

//Add ShowEventAttendees to each ShowEvent
//attendeesList is a variable containing a List of ShowEventAttendees
foreach (ShowEvent ev in show.ShowEvents)
{
foreach (ShowEventAttendee attendee in attendeesList)
{
ev.ShowEventAttendees.Add(new ShowEventAttendee
{
ShowEventID = ev.ShowEventID,
Name = attendee.Name
});
}
}

AppDataContext.DataContext.Shows.InsertOnSubmit(show);
AppDataContext.DataContext.SubmitChanges();

The issue occurs in the second foreach loop. If I take it out, the Show and ShowEvents get added to the database correctly.

What's the issue here?

EDIT for Jay:

I've tried this now:

AppDataContext.DataContext.Shows.InsertOnSubmit(show);
AppDataContext.DataContext.SubmitChanges();

//Add ShowEventAttendees to each ShowEvent
//attendeesList is a variable containing a List of ShowEventAttendees
foreach (ShowEvent ev in show.ShowEvents)
{
foreach (ShowEventAttendee attendee in attendeesList)
{
ev.ShowEventAttendees.Add(new ShowEventAttendee
{
ShowEventID = ev.ShowEventID,
Name = attendee.Name
});
}
}

AppDataContext.DataContext.SubmitChanges(); //trying to update, error here

But I get the same error on that last line. Going through the debugger, ShowEventID is no longer 0, but I'm getting the same error.