When performing an update operation with ReactiveMongoTemplate in Spring Boot 3.0.5, why does it result in the DuplicateKeyException being thrown?

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The DuplicateKeyException is typically thrown when attempting to insert a document with a duplicate key in a collection with a unique index. However, in this case, it can also be caused by attempting to perform an update operation that would result in a document having a duplicate key in a collection with a unique index.

To avoid this exception while performing the update operation, you should verify that the update operation does not result in a duplicate key being introduced. This can be done by specifying a unique key constraint on the collection and adding a check to the update query that ensures that the new values being added to the document do not result in a duplicate key. Additionally, you can also use the "upsert" option in the update query to perform an insert operation automatically if the document does not exist.

Alternatively, you can catch and handle the DuplicateKeyException by taking appropriate action, such as rolling back the transaction or informing the user that the update operation could not be completed due to a conflict with an existing document.