title: Pandas - Remove duplicates across multiple columns
body: <p>I am trying to efficiently remove duplicates in Pandas in which duplicates are inverted across two columns. For example, in this data frame:</p>
  
  <pre><code>import pandas as pd
  key = pd.DataFrame({'p1':['a','b','a','a','b','d','c'],'p2':['b','a','c','d','c','a','b'],'value':[1,1,2,3,5,3,5]})
  df = pd.DataFrame(key,columns=['p1','p2','value'])
  print frame
  
         p1 p2 value
      0  a  b    1
      1  b  a    1
      2  a  c    2
      3  a  d    3
      4  b  c    5
      5  d  a    3
      6  c  b    5
  </code></pre>
  
  <p>I would want to remove rows 1, 5 and 6, leaving me with just:</p>
  
  <pre><code>      p1 p2 value
      0  a  b    1
      2  a  c    2
      3  a  d    3
      4  b  c    5
  </code></pre>
  
  <p>Thanks in advance for ideas on how to do this.</p>