Document dublication on a single submission

I have a collection that should have a unique value in addition the id, like this

  book: { type: String, unique: true },

```When I check on the db, `book`  is not indexed, so I tried indexing it to ensure there is no value duplication of `book` but I get an error, even on unique values not in the database.

MongoServerError: E11000 duplicate key error collection: test.library index: book_1 dup key: { book: null }

I am writing to the collection I want to only when I remove an index on book. But this makes us have a two documents on a single submit, first with some fields, which some are empty and the second one which has all fields as expected.