checking dropdown list values while inserting new records in database

Hi,
I  have Products page in this .i have  two dropdownlists(ddlBrand , ddlItems ) in datagrids edit command column, and i am  populating  those from two diffrent classes
like
' Products.aspx page load event


TempDS = ObjBrands.getAll
TempDS = ObjProducts.getAll


using  my Products page i wil insert new records  in new table.

my conditions is

Brnads table has , sony,LG
Items tables has, cameras, laptops ,Lcds

in proucts page i am populating brands table data in ddlBrand dropdown and Items data in ddlItems dropdown

here  for  example i shuld only save those records like i select  sony from ddlBrand and  Cameras from ddlItems
if i select again this combination i shuld display a message to user ,  u want't select this combination  u already selected!


 
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