Does a MongoDB multikey index return duplicate parent documents if multiple array elements match?

Working with Data
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I’m working with a MongoDB collection where documents contain an array of subdocuments. For example:

{
  _id: 1,
  items: [
    { type: "A", value: 10 },
    { type: "B", value: 20 }
  ]
}

I’ve created an index on the subdocument field:

db.test.createIndex({ "items.type": 1 })

Since items is an array of subdocuments, this is a multikey index. My understanding is that MongoDB will create multiple index keys for a single parent document (e.g., one for “A”, one for “B”).

My question is:

  • When a query uses this index, and multiple array elements match, does MongoDB return duplicate parent documents?

  • For example:

db.test.find({ "items.type": { $in: ["A", "B"] } })

Will the result contain _id: 1 only once, or will it appear twice since two index keys match?

I’m trying to confirm how MongoDB handles deduplication when scanning multikey indexes.