Problems when accessing MultiIndex level with duplicated level names
#### Code Sample, a copy-pastable example if possible

```python
In [2]: mi = pd.MultiIndex.from_product([['i'], ['ii'], ['iii']])

In [3]: mi.rename([1,5,6]).get_level_values(1) # Interpreted as label
Out[3]: Index(['i'], dtype='object', name=1)

In [4]: mi.rename([1,5,1]).get_level_values(1) # Interpreted as index
Out[4]: Index(['ii'], dtype='object', name=5)

In [5]: mi.rename(['a',5,'a']).get_level_values('a') # ValueError is OK, KeyError is not
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
/home/nobackup/repo/pandas/pandas/core/indexes/multi.py in _get_level_number(self, level)
    670                 raise ValueError('The name %s occurs multiple times, use a '
--> 671                                  'level number' % level)
    672             level = self.names.index(level)

ValueError: The name a occurs multiple times, use a level number

During handling of the above exception, another exception occurred:

KeyError                                  Traceback (most recent call last)
<ipython-input-5-e8a616f9610f> in <module>()
----> 1 mi.rename(['a',5,'a']).get_level_values('a')

/home/nobackup/repo/pandas/pandas/core/indexes/multi.py in get_level_values(self, level)
    975         Index(['d', 'e', 'f'], dtype='object', name='level_2')
    976         """
--> 977         level = self._get_level_number(level)
    978         values = self._get_level_values(level)
    979         return values

/home/nobackup/repo/pandas/pandas/core/indexes/multi.py in _get_level_number(self, level)
    673         except ValueError:
    674             if not isinstance(level, int):
--> 675                 raise KeyError('Level %s not found' % str(level))
    676             elif level < 0:
    677                 level += self.nlevels

KeyError: 'Level a not found'

In [6]: mi.rename([1, 'a', 'a']).get_level_values(1) # How am I going to access the second level?!
Out[6]: Index(['i'], dtype='object', name=1)

```

#### Problem description

There are different problems, but the root cause is (I think) the same:
1. the first is trivial: the ``KeyError`` in ``In [5]:`` should not appear
2. the second is that the interpretation of an integer changes when there is a duplicate name (difference between ``Out[3]`` and ``Out[4]``)
3. the third is that in a situation like ``In [6]``, I have no way whatsoever to access the second column, since it is denoted by a duplicated name _and_ its index is also the name of another column (sure, this is a perverse example, but I suspect it can bite in some cases in which users use duplicate labels _and_ pandas internal code adds integer labels)


#### Expected Output

If we were to design this from scratch, the solution would be simple: prioritize the "index" interpretation of an integer over the "label" interpretation, so that the former is always unambiguous. Is this a too strong API change?

If the answer is "no", I will be happy to implement it, possibly with a temporary warning in those cases where the behaviour will change (that is: requested label is integer _and_ is present in the names).

If the answer is "yes", I would like to at least suppress the ``KeyError`` in ``In [5]:`` and have ``In [4]:`` raise an error rather than return a result inconsistent with ``In [3]:``.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: 04db779d4c93d286bb0ab87780a85d50ec490266
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-4-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: it_IT.UTF-8
LOCALE: it_IT.UTF-8

pandas: 0.22.0.dev0+388.g04db779d4
pytest: 3.2.3
pip: 9.0.1
setuptools: 36.7.0
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.0dev
tables: 3.3.0
numexpr: 2.6.1
feather: 0.3.1
matplotlib: 2.0.0
openpyxl: 2.3.0
xlrd: 1.0.0
xlwt: 1.3.0
xlsxwriter: 0.9.6
lxml: 4.1.1
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.2.1


</details>
