DataFrame.nlargest does not handle duplicate values
#### Code Sample, a copy-pastable example if possible

```python
# Your code here

df = pandas.DataFrame({'a': [1, 10, 8, 11, -1],
                'b': [1.0, 2.0, 3.0, 3.0, 4.0]})
df.nlargest(3, 'b')
Result:
	a	b
4	-1	4.0
2	8	3.0
3	11	3.0
2	8	3.0
3	11	3.0
```
#### Problem description

[this should explain **why** the current behaviour is a problem and why the expected output is a better solution.]

Duplicate values in column 'b' is not being handled correctly. The result is different than 
what "df.sort_values('b', ascending=False).head(3)" produces.


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#### Expected Output

	 a	b
4	-1	4.0
2	8	3.0
3	11	3.0

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

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Linux
OS-release: 4.3.5-smp-808.15.0.0
machine: x86_64
processor: 
byteorder: little
LC_ALL: en_US.UTF-8
LANG: None
LOCALE: None.None

pandas: 0.19.2
nose: None
pip: None
setuptools: None
Cython: None
numpy: 1.11.1
scipy: 0.15.1
statsmodels: 0.6.1
xarray: None
IPython: 2.0.0
sphinx: None
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: 3.1.1
numexpr: 2.5
matplotlib: 1.5.2
openpyxl: None
xlrd: 0.9.3
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0b8
httplib2: 0.9.2
apiclient: 1.5.5
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
pandas_datareader: None

[paste the output of ``pd.show_versions()`` here below this line]

</details>
