title: select rows with unidentical column values using R
body: <p>I need to create a new data frame that excludes dams that appear in "dam1" and
  "dam2" columns on the same fosdate (fostering date). I tried <code>df &lt;- df[df$dam1!=df$dam2,]</code> but did not work. Dam1 and dam2 are factors which are the id's of mothers. </p>
  
  <p>my df:</p>
  
  <pre><code>fosdate      dam1     dam2
  8/09/2009    2Z523    2Z523
  30/10/2009   1W509    5C080
  30/10/2009   1W509    5C640
  30/10/2009   1W509    1W509
  1/10/2009    1W311    63927
  </code></pre>
  
  <p>The new data frame that I need to get is:
  dfnew:</p>
  
  <pre><code>fosdate      dam1     dam2
  30/10/2009   1W509    5C080
  30/10/2009   1W509    5C640
  1/10/2009    1W311    63927
  </code></pre>
  
  <p>Would appreciate any help!</p>
  
  <p>Bazon</p>