Intermittent KeyErrors for Series with duplicates in string index
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
s1 = pd.Series(data = [1,2,3,4,5], index = [1,2,3,4,4])
s2 = pd.Series(data = [1,2,3,4,5], index = ['1', '2', '3', '4', '4'])

s1[0] # KeyError
s2[0] # KeyError

s1.iloc[:4][0] # KeyError
s1.iloc[:5][0] # KeyError

s2.iloc[:4][0] # Returns 1!
s2.iloc[:5][0] # TypeError & KeyError
```
#### Problem description

``s2[0]`` throws a KeyError because the key, 0, is not in the index. This behavior is expected. ``s2.iloc[:5][0]`` also has this correct behavior. However, once the duplicate index has been removed, ``s2.iloc[:4][0]``, this error is not thrown and instead the value at **position** 0 is returned. 

This error is present with string type indexes, but does not occur with integer indexes (see examples involving ``s1`` above). I have not tried other index types. 

Issue is present in a fresh conda environment with python version 3.6.2 and pandas version 0.21.1 (via conda-forge). 

#### Expected Output

``s2.iloc[:4][0]`` should fail with TypeError and KeyError. 

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.2.final.0
python-bits: 64
OS: Linux
OS-release: 2.6.32-696.13.2.el6.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.21.1
pytest: None
pip: 9.0.1
setuptools: 36.4.0
Cython: None
numpy: 1.13.1
scipy: None
pyarrow: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
