title: Filter duplicates MySQL
body: <p>I am not exactly sure of the best way to explain my issue. 
  I have a table similar to this</p>
  
  <pre><code>+---------+------------+---------------+--+
  | user_id | invoice_no |  invoiced_hrs |  |
  +---------+------------+---------------+--+
  |    1    |    1023    |    8.1234     |  |
  |    2    |    1023    |    1.1234     |  |
  |    1    |    1023    |    5.8765     |  |
  |    4    |    1085    |    6.7654     |  |
  |    4    |    1085    |    7.5435     |  |
  |    6    |    1022    |    5.6789     |  |
  +---------+------------+---------------+--+
  </code></pre>
  
  <p>I need to be able to filter my query without having multiple user_id's associated with any invoice_no. Duplicate invoice numbers are OK so long as there is only one user_id associated with it. </p>
  
  <p>What I am attempting to do is to keep only the rows from the user who has the majority of the hours on the invoice.</p>