How to disable a dropdown option in selected once

<html>
<select>
  <option value="volvo">Volvo</option>
  <option value="saab">Saab</option>
  <option value="mercedes">Mercedes</option>
  <option value="audi">Audi</option>
</select>
<input type=submit name=submit value=add>
</html>

now if i select volvo after adding a row next time the volvo should be disabled

what does this mean? even your form is invalid.

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