Find dupes in mongo

// Desired unique index: 
// db.categories.ensureIndex({ category_url: 1, category_page_url: 1 }, { unique: true})
 
db.categories.aggregate([
  { $group: { 
    _id: { category_url: "$category_url", category_page_url: "$category_page_url" }, 
    uniqueIds: { $addToSet: "$_id" },
    count: { $sum: 1 } 
  }}, 
  { $match: { 
    count: { $gt: 1 } 
  }}
])