[BUG] [Java]Specified ContentType header values generates uncompilable code

[tejasmehta269]

Description

Error like below.
Error:(126,25) java: variable contentType is already defined in method

openapi-generator version
4.1.0, 4.1.3, 5.1.1

OpenAPI declaration file content or url.

• name: Content-Type
  in: header
  description: Replace 'json' with 'xml' if preferred
  required: true
  style: simple
  schema:
  type: string
  example: application/json

OpenAPI declaration file content or url

Command line used for generation

Steps to reproduce

Used Maven plugin to re-produce issue.
org.openapitools
openapi-generator-maven-plugin
5.1.1

Related issues/PRs

Suggest a fix/enhancement

Simillar issue resovled earlier here. #7101