title: Avoid duplicates in bigquery
body: <p>i'm working with bigquery and in the documentation it's said:</p>
  
  <blockquote>
    <p>Unlike a traditional RDBMS, there is no notion of primary/secondary or
    row-id keys. If required, identify a column in the table schema for
    that purpose.</p>
  </blockquote>
  
  <p>Do you know how could i insert without duplicates as a pk would do(but not only in the same insert)?
  Regards</p>