No Issue - Add a warn in the logs when elements have duplicated vars

[ricardozanini]

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In this PR, we add a WARN to the logs when the OpenAPI file has duplicated element variables. For example:

    CheckpointInfo:
      type: object
      properties:
        checkpointId:
          type: integer
          format: int64
        failureCause:
          $ref: '#/components/schemas/SerializedThrowable'
    SavepointInfo:
      type: object
      properties:
        failure-cause:
          $ref: '#/components/schemas/SerializedThrowable'
        location:
          type: string

Will fail because failure-cause and failureCause are the same variables from the Java syntax perspective as soon as we convert both to CamelCase.

Why not remove the duplicates?

Because users will only realize this bug in runtime when it's too late, and Json marshaling won't work:

        /**
        * Get failureCause
        * @return failureCause
        **/
        @com.fasterxml.jackson.annotation.JsonProperty("failure-cause")
        public SerializedThrowable getFailureCause() {
            return failureCause;
        }

        /**
        * Get failureCause
        * @return failureCause
        **/
        @com.fasterxml.jackson.annotation.JsonProperty("failureCause")
        public SerializedThrowable getFailureCause() {
            return failureCause;
        }

Users should fix the OpenAPI spec on their side to have a healthy codegen.

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