title: rbind data frames, duplicated rownames issue
body: <p>While duplicated row (and column) names are allowed in a <code>matrix</code>, they are not allowed in a <code>data.frame</code>. Trying to <code>rbind()</code> some data frames having row names in common highlights this problem. Consider two data frames below:</p>
  
  <pre><code>foo = data.frame(a=1:3, b=5:7)
  rownames(foo)=c("w","x","y")
  bar = data.frame(a=c(2,4), b=c(6,8))
  rownames(bar)=c("x","z")
  # foo               bar
  #   a b               a b
  # w 1 5             x 2 6
  # x 2 6             y 4 8
  # y 3 7
  </code></pre>
  
  <p>Now trying to <code>rbind()</code> them (Pay attention to the row names):</p>
  
  <pre><code>rbind(foo, bar)
  #    a b
  # w  1 5
  # x  2 6
  # y  3 7
  # x1 2 6
  # z  4 8
  </code></pre>
  
  <p>But for the case of <code>matrix</code>:</p>
  
  <pre><code>rbind(as.matrix(foo), as.matrix(bar))
  #   a b
  # w 1 5
  # x 2 6
  # y 3 7
  # x 2 6
  # z 4 8
  </code></pre>
  
  <p>Here is the problem: How to <code>rbind()</code> two data frames, having duplicated rows (with the same row name) removed? </p>