Why do I get this exception? {An item with the same key has already been added."})

Aknittel

NewSellerID is the result of a lookup on tblSellers. These tables (tblSellerListings and tblSellers) are not "officially" joined with a foreign key relationship, either in the model or in the database, but I want some referential integrity maintained for the future. So my issue remains. Why do I get the exception ({"An item with the same key has already been added."}) with this code, if I don't begin each iteration of the foreach loop with a new ObjectContext and end it with SaveChanges, which I think will affect performance. Also, could you tell me why ORCSolutionsDataService.tblSellerListings (An ADO.NET DataServices/WCF object is not IDisposable, like LINQ to Entities??

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// Add listings to previous seller
 int NewSellerID = 0;
 
 // Look up existing Seller key using SellerUniqueEBAYID
 var qryCurrentSeller = from s in service.tblSellers
 					   where s.SellerEBAYUserID == SellerUserID
 					   select s;
 
 foreach (var s in qryCurrentSeller)
 	NewSellerID = s.SellerID;
 
 // Save the selected listings for this seller
 
 foreach (DataGridViewRow dgr in dgvRows)
 {
 
 	ORCSolutionsDataService.tblSellerListings NewSellerListing = new ORCSolutionsDataService.tblSellerListings();
 	NewSellerListing.ItemID = dgr.Cells["txtSellerItemID"].Value.ToString();
 	NewSellerListing.Title = dgr.Cells["txtSellerItemTitle"].Value.ToString();
 	NewSellerListing.CurrentPrice = Convert.ToDecimal(dgr.Cells["txtSellerItemPrice"].Value);
 	NewSellerListing.QuantitySold = Convert.ToInt32(dgr.Cells["txtSellerItemSold"].Value);
 	NewSellerListing.EndTime = Convert.ToDateTime(dgr.Cells["txtSellerItemEnds"].Value);
 	NewSellerListing.CategoryName = dgr.Cells["txtSellerItemCategory"].Value.ToString();
 	NewSellerListing.ExtendedPrice = Convert.ToDecimal(dgr.Cells["txtExtendedReceipts"].Value);
 	NewSellerListing.RetrievedDtime = Convert.ToDateTime(dtSellerDataRetrieved.ToString());
 	NewSellerListing.SellerID = NewSellerID;
 
 	service.AddTotblSellerListings(NewSellerListing);
 
 }
 
 service.SaveChanges();
 

}

catch (Exception ex)
{
MessageBox.Show("Unable to add a new case. Exception: " + ex.Message);

}

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