Assigning a string index as a column once can make further assignments affect both index and column.
If you assign the dataframe's index to as a column as well, without resetting the index, any subsequent assignments on the column seem to affect both the column as well as the index.
The behaviour is there for Index, Int64Index, and Float64Index but not DatetimeIndex.
#### Code Sample, a copy-pastable example if possible.

``` python
>>> import pandas as pd
>>> df=pd.DataFrame(index=['A', 'B', 'C'])
>>> df['X']=df.index
>>> df
   X
A  A
B  B
C  C
>>> df['X']=['x', 'y', 'z']
>>> df
   X
x  x
y  y
z  z
>>> df.ix[0, 'X']='q'
>>> df
   X
q  q
y  y
z  z
```
#### Expected Output

``` python
   X
A  x
B  y
C  z
```
#### output of `pd.show_versions()`

``` python
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 58 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.18.1
nose: 1.3.7
pip: 8.1.2
setuptools: 23.0.0
Cython: 0.24
numpy: 1.10.4
scipy: 0.17.1
statsmodels: None
xarray: 0.7.2
IPython: None
sphinx: None
patsy: None
dateutil: 2.4.1
pytz: 2016.4
blosc: None
bottleneck: 1.0.0
tables: 3.2.2
numexpr: 2.5.2
matplotlib: 1.5.1
openpyxl: 2.3.2
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.13
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.40.0
pandas_datareader: None
```
