title: unique() for more than one variable
body: <p>I have the following data frame in R:</p>
  
  <pre><code>&gt; str(df)
  'data.frame':   545227 obs. of  15 variables:
   $ ykod : int  93 93 93 93 93 93 93 93 93 93 ...
   $ yad  : Factor w/ 42 levels "BAKUGAN","BARBIE",..: 30 30 30 30 30 30 30 30 30 30 ...
   $ per  : Factor w/ 3 levels "2 AYLIK","3 AYLIK",..: 3 3 3 3 3 3 3 3 3 3 ...
   $ donem: int  201101 201101 201101 201101 201101 201101 201101 201101 201101 201101 ...
   $ sayi : int  201101 201101 201101 201101 201101 201101 201101 201101 201101 201101 ...
   $ mkod : int  4 5 9 11 12 18 20 22 25 26 ...
   $ mad  : Factor w/ 10464 levels "   Defne Market          ",..: 405 8075 9710 10145 9297 7973 2542 3892 2759 5769 ...
   $ mtip : Factor w/ 29 levels "Abone Bürosu                                      ",..: 2 20 20 2 2 2 2 2 2 2 ...
   $ kanal: Factor w/ 2 levels "OB","SS": 2 2 2 2 2 2 2 2 2 2 ...
   $ bkod : int  110565 110565 110565 110565 110565 110565 110565 110565 110565 110565 ...
   $ bad  : Factor w/ 212 levels "4. Levent","500 Evler",..: 167 167 167 167 167 167 167 167 167 167 ...
   $ bolge: Factor w/ 12 levels "Adana Şehiriçi",..: 7 7 7 7 7 7 7 7 7 7 ...
   $ sevk : int  2 3 3 3 2 2 2 6 2 2 ...
   $ iade : int  2 1 0 2 0 2 1 0 0 2 ...
   $ satis: int  0 2 3 1 2 0 1 6 2 0 ...
  </code></pre>
  
  <p>I want to list unique (like SQL's DISTINCT) values for selected multiple variables. For example, <code>unique(yad)</code> gives me the names of each 42 elements, but I need to extract two columns (<code>yad</code> and <code>per</code> together, with all unique combinations):</p>
  
  <pre><code>yad           per
  ---           ---
  BARBIE        AYLIK
  BAKUGAN       2 AYLIK
  MICKEY MOUSE  2 AYLIK
  TINKERBELL    3 AYLIK
  ...           ...
  </code></pre>
  
  <p>How can I achieve this?</p>