Insert into table if record don't exist

Question

hi,

The below is the sql statement that inserts the record if only the record is not exists in table.. but the statement i written is inserting multiple times the same record... please help me to resolve my problem..

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INSERT INTO TableName(AccountNo,Customer,ContactNo) select 'AP1234','Saketh','984822338' FROM TableName WHERE NOT EXISTS(Select * From TableName Where AccountNo='AP1234');