Duplicate documents with unionWith

Last 20260116.md journal, I mentioned that $unionWith may result in duplicate documents. In the example pipeline below, it produced duplicate documents (data are mock/generated). This is because we’re basically just combining the results of the two pipelines. Note that you need to use $sort with the identifier so that you can see them adjacent. Without sorting, the documents from another pipeline may be far apart from the first.

[
  {
    $match: {
      gender: { $regex: ".*ale" }
    }
  },
  {
    $unionWith: {
      coll: "persons",
      pipeline: [
        {
          $match: {
            gender: { $regex: ".*le" }
          }
        }
      ]
    }
  },
  { $sort: { _id: 1 } }
]

Sample result

[
	{
		"_id": {
			"$oid": "696e23bbbea06b125b46d15e"
		},
		"id": 2,
		"first_name": "Ashlee",
		"last_name": "Alchin",
		"email": "aalchin1@fema.gov",
		"gender": "Female",
		"ip_address": "157.142.117.33"
	},
	{
		"_id": {
			"$oid": "696e23bbbea06b125b46d15e"
		},
		"id": 2,
		"first_name": "Ashlee",
		"last_name": "Alchin",
		"email": "aalchin1@fema.gov",
		"gender": "Female",
		"ip_address": "157.142.117.33"
	}
]