`allOf` does not filter duplicate properties

[hauner]

a schema with allOf and properties with the same name

  schema:
    allOf:
      - type: object
        properties:
          foo:
            type: string
      - type: object
        properties:
          foo:
            type: string

will generate a class with two foo properties

from: openapi-processor/openapi-processor-maven#2

[hauner]

.. there are more issues hiding if foo has different types etc.