title: Remove all unique rows
body: <p>I am trying to figure out how to remove all unique rows, from a data frame, but if it has a duplicate, I want that to stay in.  For Example - I want all columns from this with col1 the same:</p>
  
  <pre><code>df&lt;-data.frame(col1=c(rep("a",3),"b","c",rep("d",3)),col2=c("A","B","C",rep("A",3),"B","C"),col3=c(3,3,1,4,4,3,2,1))
  df
    col1 col2 col3
  1    a    A    3
  2    a    B    3
  3    a    C    1
  4    b    A    4
  5    c    A    4
  6    d    A    3
  7    d    B    2
  8    d    C    1
  
  subset(df,duplicated(col1))
    col1 col2 col3
  2    a    B    3
  3    a    C    1
  7    d    B    2
  8    d    C    1
  </code></pre>
  
  <p>But I want to have rows 1,2,3,6,7,8 since they all have the same col 1.  How do I get 1 and 6 to be included?  Or, conversely, how do I remove rows that do not have a duplicate?</p>