I have the java configuration for the Spring Data Elaticsearch(using Transport Client) and ESTemplate. Here some except:

@Configuration
@EnableElasticsearchRepositories(basePackages = "subpackage-in-this-project")
@PropertySource("file:path-to-file")
public class ESConfig {

    @Bean
    ElasticsearchTemplate elasticsearchTemplate(Client client) {
        return new ElasticsearchTemplate(client);
    }

    @Bean
    Client client() { 
// configuration of the ES client
   }

}

And I have a config that extends the one above in the different project.

@Configuration
@ComponentScan("package-prefix-that-matches-packages-in-both-projects")
@EnableElasticsearchRepositories(basePackages = "subpackage-in-this-project")
@PropertySource("file:same-path-to-file-as-in-the-config-above")
public class ExtendedESConfig extends ESConfig {

    @Value("index-name")
    private String indexName;

    @Bean
    public String indexName() {
        return indexName;
    }
}

Upon executing a third Spring Boot application, which uses the dependency on the project with ExtendedESConfig, I get this and I can't quite understand why does it happen, started to happen after upgrading to 2.2.9.RELEASE from 2.0.5.RELEASE Spring Boot version.

***************************
APPLICATION FAILED TO START
***************************

Description:

The bean 'elasticsearchTemplate', defined in class path resource [my/package/ESConfig.class], could not be registered. A bean with that name has already been defined in class path resource [my/other/package/ExtendedESConfig.class] and overriding is disabled.

Action:

Consider renaming one of the beans or enabling overriding by setting spring.main.allow-bean-definition-overriding=true

2020-08-30 16:49:46 ERROR [main] org.springframework.boot.diagnostics.LoggingFailureAnalysisReporter:40 - 

Important remark from my comment:

... sadly, I am not the one who wrote this ES config and built the whole infrastructure around it. ...

At the time of this question, I don't own ExtendedESConfig nor can change it.

spring.main.allow-bean-definition-overriding=true is NOT an answer as I don't want to override bean declarations.