[Python]Generated code has naming conflict

Description

The generated Python code renamed the camelCase property to under_score format and conflict with another property.

Swagger-codegen version

swagger-codegen-cli-2.4.14

Swagger declaration file content or url

definitions:
  RestrictedBanInfo:
    type: object
    required:
      - ban_type
    properties:
      ban_type:
        type: string
      banType:
        type: string
        description: the same value as ban_type
        readOnly: true

Command line used for generation

java -jar swagger-codegen-cli-2.4.14.jar generate -i swagger.json -g python --library asyncio -o ./

Steps to reproduce

1. define an object with 2 properties. Using the camelCase and under_score case to name those 2 properties. e.g. xY and x_y
2. generate python client code
3. check generated model code

class RestrictedBanInfo(object):
    openapi_types = {
        'ban_type': 'str',
        'ban_type': 'str',
     }

    attribute_map = {
        'ban_type': 'ban_type',
        'ban_type': 'banType',
    }

    def __init__(self, ban_type=None, ban_type=None):
        ...

Reproduce the issue with the RestrictedBanInfo YAML definition and the swagger-codegen-cli-2.4.14 Python asyncio generation command. Inspect the generated model's openapi_types, attribute_map, and init; done means both ban_type and banType remain distinct properties without duplicate dictionary keys or constructor arguments.