gistfile1.txt

         <option value="1" selected >1</option>
        
      
        
          <option value="2"  >2</option>
        
      
        
          <option value="3" selected >3</option>
        
      
        
          <option value="4"  >4</option>
        
      
        
          <option value="5"  >5</option>
        
      
        
          <option value="6"  >6</option>
        
      
        
          <option value="7"  >7</option>
        
      
        
          <option value="8"  >8</option>
        
      
        
          <option value="9"  >9</option>
        
      
        
          <option value="10"  >10</option>
        
      
        
          <option value="11"  >11</option>
        
      
        
          <option value="12"  >12</option>
        
      
        
          <option value="13" selected >13</option>

Shouldn't this be a == rather than a in:

{% if select.value in field.value %}

https://github.com/tomchristie/django-rest-framework/blob/f601c6c1c36a3450f4047b10d3b0955a511080a0/rest_framework/templates/rest_framework/inline/select_multiple.html#L18

http://postimg.org/image/unui36otn/