Best way to check for duplicates before inserting or updating record in linq to sql ?

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LINQ

Hi ,

My question is which is the best practise to check for duplication before inseerting or updating using linq to sql ,

some says use singleordefault and then check for null ,
someboday says user firstordefault ,
some says use .any which returns boolean ,
some says use count ,..

I know all this are the correct way , but i want to know which is the best practise for this