Here is a sample stored procedure that prevents duplicate inserts with the same value by first performing a test using SQL Server's EXISTS statement.

IF OBJECT_ID('prcTagNameInsert') IS NOT NULL
    DROP PROCEDURE prcTagNameInsert
GO
CREATE PROCEDURE prcTagNameInsert
    @tagName    VARCHAR(50)
AS

IF NOT EXISTS(SELECT 1 FROM Tag WHERE tagName = @tagName)
    INSERT INTO Tag (tagName) 
    VALUES (@tagname)

Labels: SQL