For your case, you can see if there is a record with 112 as a value before adding 112 as column value in a new record.
SELECT Barcode WHERE Barcode = 112
This would do the trick. You can pass the 112 as a parameter (
but that is a different story). Otherwise, in my own recommendation it is better to use a primary key and set that column to identity so that the database engine itself ensures that a duplicate data is not entered, and if that happens, an error is raised.