How to define different responses for same HTTP status code in OpenAPI (Swagger)?

This is possible in OpenAPI 3.0 but not in 2.0.

OpenAPI 3.0 supports oneOf for specifying alternate schemas for the response. You can also add multiple response examples, such as successful and failed responses. Swagger UI supports multiple examples since v. 3.23.0.

openapi: 3.0.0
...

paths:
  /something:
    get:
      responses:
        '200':
          description: Result
          content:
            application/json:
              schema:
                oneOf:
                  - $ref: '#/components/schemas/ApiResultOk'
                  - $ref: '#/components/schemas/ApiResultError'
              examples:
                success:
                  summary: Example of a successful response
                  value:
                    result: true
                    token: abcde12345
                error:
                  summary: Example of an error response
                  value:
                    result: false
                    errorCode: "00002"
                    errorMsg: "duplicated account already exist"

components:
  schemas:
    ApiResultOk:
      type: object
      properties:
        result:
          type: boolean
          enum: [true]
        token:
          type: string
      required:
        - result
        - token
    ApiResultError:
      type: object
      properties:
        result:
          type: boolean
          enum: [false]
        errorCode:
          type: string
          example: "00002"
        errorMsg:
          type: string
          example: "duplicated account already exist"

In OpenAPI/Swagger 2.0, you can only use a single schema per response code, so the most you can do is define the varying fields as optional and document their usage in the model description or operation description.

swagger: "2.0"
...

definitions:
  ApiResult:
    type: object
    properties:
      result:
        type: boolean
      token:
        type: string
      errorCode:
        type: string
      errorMsg:
        type: string
    required:
      - result