Weird behavior using nlargest/nsmallest when there are the n smallest/largest values are identical
#### Code Sample, a copy-pastable example if possible

```
python -c "import pandas as pd; df = pd.DataFrame(dict(a=[1, 1, 2, 3], b=[1, 2, 3, 4])); print(df.nsmallest(2, 'a'))"
```

#### Problem description

When using nlargest/nsmallest and the n largest / smallest values are identical, the method seems to return the dataframe concatenated with the filtered version of itself.
Furthermore if all values are identical, you get the full dataframe concatenated with itself, regardless of the choice of `n`

#### Expected Output

Not really sure, I guess in the example above you should simply get a dataframe that looks like this
```pd.DataFrame(dict(a=[1, 1], b=[1, 2]))```
however if you were to have 
``` df = pd.DataFrame(dict(a=[1, 1, 1, 1], b=[1, 2, 3, 4]))```
and asked for
```df.nlargest(2, 'a')``` you should again get 
```pd.DataFrame(dict(a=[1, 1], b=[1, 2]))```

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Linux
OS-release: 4.8.0-34-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_IE.UTF-8
LOCALE: None.None

pandas: 0.19.2
nose: 1.3.7
pip: 9.0.1
setuptools: 28.3.0
Cython: 0.23.4
numpy: 1.12.0
scipy: 0.16.1
statsmodels: 0.6.1
xarray: None
IPython: None
sphinx: 1.3.1
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: None
tables: 3.2.0
numexpr: 2.4.6
matplotlib: 1.5.0
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: 0.9.2
apiclient: None
sqlalchemy: 1.0.9
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.38.0
pandas_datareader: None
</details>
