[BUG][JAVA] Generated model contains double JsonTypeName annotation

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• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

The attached spec seems to be causing some problems with version 5.0.0 of the openapi-generator-maven-plugin. It creates this model class, which has two JsonTypeName annotations, causing a compilation error:

@JsonPropertyOrder({
  First.JSON_PROPERTY_PROP
})
@JsonTypeName("first")
@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", date = "2021-01-28T07:47:45.336926+01:00[Europe/Oslo]")
@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.EXISTING_PROPERTY, property = "discriminator", visible = true)
@JsonTypeName("first")

public class First extends Parent {
...
}

generated-sources/openapi/src/main/java/test/model/First.java:[40,1] com.fasterxml.jackson.annotation.JsonTypeName is not a repeatable annotation type

openapi-generator version

openapi-generator-maven-plugin, version 5.0.0

OpenAPI declaration file content or url

https://gist.github.com/andersflemmen/8e4eddc90b7293fd55f4d75c637b3502

Generation Details

Maven configuration

<plugin>
    <groupId>org.openapitools</groupId>
    <artifactId>openapi-generator-maven-plugin</artifactId>
    <version>5.0.0</version>
    <executions>
        <execution>
            <goals>
                <goal>generate</goal>
            </goals>
            <phase>generate-sources</phase>
            <configuration>
                <inputSpec>${project.basedir}/openapi.json</inputSpec>
                <generatorName>java</generatorName>
                <configOptions>
                    <dateLibrary>java8</dateLibrary>
                    <feignVersion>10.x</feignVersion>
                    <java8>true</java8>
                </configOptions>
                <modelPackage>test.model</modelPackage>
                <apiPackage>test.api</apiPackage>
                <invokerPackage>test.invoker</invokerPackage>
                <generateApiTests>false</generateApiTests>
                <generateModelTests>false</generateModelTests>
                <generateSupportingFiles>true</generateSupportingFiles>
                <library>feign</library>
                <addCompileSourceRoot>true</addCompileSourceRoot>
            </configuration>
        </execution>
    </executions>
</plugin>

Reproduce the generated-sources/openapi/src/main/java/test/model/First.java output using the linked OpenAPI declaration and the shown openapi-generator-maven-plugin configuration. Start by tracing how the Java generator handles discriminator annotations and compare the duplicate JsonTypeName entries. Done means the generated model compiles with only one JsonTypeName annotation.