title: Count the number of duplicate for a column
body: <p>My objective is to get a count on how many duplicate are there in a column.<br/>So i have a column of 3516 obs. of 1 variable,<br/> there are all dates with about 144 duplicate each from 1/4/16 to 7/3/16.<br/> Example:(i put 1 duplicate each for example sake)<br/>1/4/16<br/>1/4/16<br/>31/3/16<br/>31/3/16<br/>30/3/16<br/>30/3/16<br/>29/3/16<br/>29/3/16<br/>28/3/16<br/>28/3/16<br/>so i used the function date = count(date)<br/>where date is my df date.<br/>But once i execute it my date sequence is not in order anymore. <br/> Hope someone can solve my problem.</p>