title: Finding and mapping duplicates in a pandas groupby object
body: <p>I have 2 columns, User_ID and Item_ID. Now I want to make a new column 'Reordered' which will contain values as either 0 or 1. 0 is when a particular user has ordered an item only once, and 1 is when a particular user orders an item more than once. 
  I think this can be done by grouping on User_ID and then using apply function to map duplicated items as 1 and non duplicated as 0 but I'm not able to figure out the correct python code for that.
  If someone can please help me with this. </p>