[JAVA] Nested ref generates extra class definition import

Description

I have a couple of API describing files that share the same data placed in other files. When schema definition references are nested an import of non-existing class and reference to it is created in model package although the intended model class is already there.
Please see the gist to see the mangled import and property type.

Swagger-codegen version

3.0.13

Swagger declaration file content or url

Top-level ref.yaml:

openapi: 3.0.1
info:
  title: Sample API
  description: API description in Markdown.
  version: 1.0.0
servers:
  - url: 'https://api.example.com'
paths:
  /users:
    get:
      summary: Returns a list of users.
      description: Optional extended description in Markdown.
      responses:
        '500':
          $ref: '#/components/responses/Error500'
          # References a reference in `Resposes`
components:
  schemas:
    DisplayMessage:
      $ref: './common/ref_schemas.yaml#/DisplayMessage'
    Error:
      $ref: './common/ref_errors.yaml#/Error'
  responses:
    Error500:
      $ref: './common/ref_responses.yaml#/Error500'
      # A reference to definition

The files used to reproduce the error along with created model are available as a gist

Command line used for generation

java -jar swagger-codegen-cli.jar generate -i ref.yaml -l java -o generated/clients/java

Steps to reproduce

Download the gist and run the build command

Related issues/PRs

#7797

Suggest a fix/enhancement

I assume all $refs that resolve to the same model should resolve to the same class

Start with the gist's top-level ref.yaml and referenced common files, then run the provided swagger-codegen-cli Java generation command. Compare the generated model imports and property types with the existing model classes; the fix is complete when nested $ref values resolving to the same model no longer create nonexistent duplicate class imports or references.