What happens if you insert a document with an existing unique email in a collection?

When you attempt to insert a document with an email that already exists in a collection where the email field is designated as having a unique index, the operation will result in a duplicate key error. This is because the unique index enforces the rule that no two documents can have the same value for that indexed field, which in this scenario is the email address.

When the attempt to insert the new document is made and it encounters an email that conflicts with an existing entry, MongoDB will reject the insertion and return an error indicating that a duplicate key has been detected. This behavior ensures data integrity by maintaining the uniqueness of values that must be distinct across the collection. Therefore, the outcome of the operation is explicitly handled with an error, depicting the violation of the uniqueness constraint.

In contrast, having the document inserted without issues, overwriting the existing record, or silently ignoring the operation would contradict the purpose of enforcing uniqueness within the database. Each of these alternatives would compromise the integrity of the data managed by the unique index.