aggregate.js

var results = db.sanat.aggregate([
  { $group: 
    {
            _id: { 
                   headword : "$headword",
                   senses : "$senses" 
                 },
      uniqueIds: { $addToSet: "$_id" },
          count: { $sum: 1 } 
    } 
  }, 
  { $match: 
    { count: { $gt: 1 } } 
  }
]);