<p dir="auto">Continuing <a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="12049172" data-permission-text="Title is private" data-url="https://github.com/pandas-dev/pandas/issues/3059" data-hovercard-type="pull_request" data-hovercard-url="/pandas-dev/pandas/pull/3059/hovercard" href="https://github.com/pandas-dev/pandas/pull/3059">#3059</a>.<br>
See also <a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="12194544" data-permission-text="Title is private" data-url="https://github.com/pandas-dev/pandas/issues/3092" data-hovercard-type="issue" data-hovercard-url="/pandas-dev/pandas/issues/3092/hovercard" href="https://github.com/pandas-dev/pandas/issues/3092">#3092</a></p>
<ul dir="auto">
<li>allow dupe columns when they are in the same block/dtype</li>
<li>Perhaps figure out a way to handle that case as well.</li>
</ul>
<h4 dir="auto">Code Sample, a copy-pastable example if possible</h4>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="&gt;&gt;&gt; import pandas as pd
&gt;&gt;&gt; s = pd.Series()
&gt;&gt;&gt; ts = pd.Timestamp('2016-01-01')
&gt;&gt;&gt; s['a'] = None
&gt;&gt;&gt; s['b'] = ts
&gt;&gt;&gt; s
a                   None
b    1451606400000000000
dtype: object"><pre class="notranslate"><code class="notranslate">&gt;&gt;&gt; import pandas as pd
&gt;&gt;&gt; s = pd.Series()
&gt;&gt;&gt; ts = pd.Timestamp('2016-01-01')
&gt;&gt;&gt; s['a'] = None
&gt;&gt;&gt; s['b'] = ts
&gt;&gt;&gt; s
a                   None
b    1451606400000000000
dtype: object
</code></pre></div>
<p dir="auto">OK, no worries, we got coerced to integer. Now let's just redo the <strong>same assignment</strong>:</p>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="&gt;&gt;&gt; s['b'] = ts
&gt;&gt;&gt; s
a                   None
b    2016-01-01 00:00:00
dtype: object"><pre class="notranslate"><code class="notranslate">&gt;&gt;&gt; s['b'] = ts
&gt;&gt;&gt; s
a                   None
b    2016-01-01 00:00:00
dtype: object
</code></pre></div>
<p dir="auto">That's ... suprising. This is probably just an unfortunate feature of a type inference algorithm, but it's awfully shocking.</p>
<h4 dir="auto">Related examples for testing</h4>
<p dir="auto"><a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="275727906" data-permission-text="Title is private" data-url="https://github.com/pandas-dev/pandas/issues/18410" data-hovercard-type="issue" data-hovercard-url="/pandas-dev/pandas/issues/18410/hovercard" href="https://github.com/pandas-dev/pandas/issues/18410">#18410</a><br>
<a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="324713954" data-permission-text="Title is private" data-url="https://github.com/pandas-dev/pandas/issues/21143" data-hovercard-type="issue" data-hovercard-url="/pandas-dev/pandas/issues/21143/hovercard" href="https://github.com/pandas-dev/pandas/issues/21143">#21143</a></p>
<h4 dir="auto">Expected Output</h4>
<p dir="auto">The two outputs above would be identical; I'd prefer that they were both the second form (with timestamp information preserved), but anything consistent would be better than the current state.</p>
<h4 dir="auto">output of <code class="notranslate">pd.show_versions()</code></h4>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="&gt;&gt;&gt; pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Darwin
OS-release: 15.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.1
nose: None
pip: 8.1.2
setuptools: 25.1.4
Cython: None
numpy: 1.11.1
scipy: None
statsmodels: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.5.3
pytz: 2016.6.1
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
boto: None
pandas_datareader: None"><pre class="notranslate"><code class="notranslate">&gt;&gt;&gt; pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Darwin
OS-release: 15.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.1
nose: None
pip: 8.1.2
setuptools: 25.1.4
Cython: None
numpy: 1.11.1
scipy: None
statsmodels: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.5.3
pytz: 2016.6.1
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
boto: None
pandas_datareader: None
</code></pre></div>