MS SQL tutorial on inserting into a table but checking if a duplicate entry already exists

This is a Microsoft SQL tutorial on a solution to stop inserting duplicate data into tables. This has a single code solution using NOT EXISTS to check for values before inserting.
There are numerous ways of checking if data exists before inserting but this is specific to inserting new values but checking if these values would make a duplicate record.

[bernie]

Is there an easy way to check more than 1 row

[remuneracionesorh]

I'm trying to do the same through an ODBC connection, but it doesn't recognize the "TOP" clause.