Error when I register multiple functions

Hi,

I can't register multiple functions.
`apispec.exceptions.DuplicateParameterError: Duplicate parameter with name body and location body` error occurred.

```

docs.register(foo, blueprint=base_bp.name)
docs.register(bar, blueprint=base_bp.name)

```

These are my endpoints.
```

from flask import Blueprint
from flask_apispec import doc, use_kwargs
from flask_restful import fields, marshal_with
import marshmallow as m

base_bp = Blueprint(
"base",
__name__,
url_prefix="/base"
)

request_fields = {
'name': m.fields.Str(required=True),
'address': m.fields.Str()
}

response_fields = {
'name': fields.String,
'address': fields.String,
'id': fields.Integer
}

def db_get_info(id):
return dict(name="hello", address="world", id=id)

def db_get_info2(name, address=None, id=3):
return dict(name=name, address=address, id=id)

@base_bp.route("/", methods=["GET"])
@doc(params={'id': {'description': 'test id'}})
@marshal_with(response_fields, envelope='resource')
def foo(id):
return db_get_info(id)

@base_bp.route("/params", methods=["GET"])
@doc(params={'name': {'aa': 'name'}, 'address': {'bb': 'address'}})
@use_kwargs(request_fields)
@marshal_with(response_fields, envelope='resource')
def bar(**kwargs):
return db_get_info2(**kwargs)

```

Reproduce the issue using the shown Blueprint, foo and bar endpoints, and the two docs.register calls. Start by tracing registration and parameter generation in the library; done means both functions register successfully without a DuplicateParameterError.

Written by the indexing model from the issue text.