mongodb: Find dupplicate users and save to second collection

find_dupplicate.sql

var previous_userid;
db.users.find( {"userid" : {$exists:true} }, {"userid" : 1} ).sort( { "userid" : 1} ).forEach( function(current) {

  if(current.userid == previous_userid){
    db.duplicates.update( {"_id" : current.userid}, { "$inc" : {count:1} }, true);
  }

  previous_userid = current.userid;

});