de.flapdoodle.embed.mongo: @Indexed(unique=true) does not cause Dupe Key error

When you mark a field as unique, insertion of a new document with the same value in that field should throw a duplicate key error. I can currently insert 2 documents with duplicate values via spring boot.

<parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.3.2.RELEASE</version>
</parent>
    ....
<dependency>
            <groupId>de.flapdoodle.embed</groupId>
            <artifactId>de.flapdoodle.embed.mongo</artifactId>
            <scope>test</scope>
</dependency>

Create a user class:

import org.springframework.data.annotation.Id;
import org.springframework.data.mongodb.core.index.Indexed;
import org.springframework.data.mongodb.core.mapping.Document;

@Document(collection = "appUsers")
public class User {
    @Id
    private String id;

    @Indexed(unique = true) 
    private String userName;

    @Indexed(unique = true)
    private String email;

In a test I can insert twice:

User user1 = new User("foobar","foo@bar.com"); // userName and email address in constructor
User user2 = new User("foobar","foo@bar.com"); 
userRepository.insert(user1);
userRepository.insert(user2); // expect an error here

userRepository is Autowired is a standard mongo repository:

public interface UserRepository extends MongoRepository<User,String>