title: Dont save in DB repeated rows with Django import-export
body: <p>Im importing an xls file with django import-export and all working fine, now i need delete the rows that has the same strings, i mean </p>
  
  <pre><code>id - name
  1  - Jhon
  2  - Jhon
  3  - Peter
  </code></pre>
  
  <p>Only insert in DB when importing the rows 2 and 3</p>
  
  <p>Until now, i have this:</p>
  
  <pre><code>class ProyectosResource(resources.ModelResource):
         #repeated_rows = fields.Field()
  
         class Meta:
             model = Proyectos
  
  class ProyectosAdmin(ImportExportModelAdmin):
          resource_class = ProyectosResource
  </code></pre>