title: SQL group by last name from a table with duplicate tuple
body: <p>I have a question from an assignment for database using sqlite3 or MySQL. The question is:</p>
  
  <p>Find all the last names belonging to two or more presidents. Do not repeat a last name, and remember that the same person serving two different terms (e.g., Grover Cleveland) does not constitute a case of two presidents with the same last name.</p>
  
  <pre><code>SELECT lastName FROM Presidents GROUP BY lastName HAVING COUNT(lastName) &gt; 1;
  </code></pre>
  
  <p>I only managed to separate all the unique last names, however, I cannot remove same person that may be president twice such as Grover Cleveland. 
  Can someone teach me how I can remove the same person from the query? </p>
  
  <p>I thought of including "WHERE firstName is UNIQUE", but it does not work.
  Any help is appreciated. Thanks </p>