title: Pandas join: Does not recognize joining column
body: <p>I have no idea what's happening, the title is just a first-order approximation. I'm trying to join two data frames:</p>
  
  <pre><code>&gt;&gt;&gt; df_sum.head()
           TUCASEID  t070101  t070102  t070103  t070104  t070105  t070199  \
  0  20030100013280        0        0        0        0        0        0   
  1  20030100013344        0        0        0        0        0        0   
  2  20030100013352       60        0        0        0        0        0   
  3  20030100013848        0        0        0        0        0        0   
  4  20030100014165        0        0        0        0        0        0   
  
     t070201  t070299  shopping  year  
  0        0        0         0  2003  
  1        0        0         0  2003  
  2        0        0        60  2003  
  3        0        0         0  2003  
  4        0        0         0  2003  
  &gt;&gt;&gt; emp.head()
           TUCASEID status
  0  20030100013280    emp
  1  20030100013344    emp
  2  20030100013352    emp
  4  20030100014165    emp
  5  20030100014169    emp
  </code></pre>
  
  <p>That's the data frames, I want to join them over the common column <code>TUCASEID</code>, of which there are intersections:</p>
  
  <pre><code>&gt;&gt;&gt; np.intersect1d(emp.TUCASEID, df_sum.TUCASEID)
  array([20030100013280, 20030100013344, 20030100013352, ..., 20131212132462,
         20131212132469, 20131212132475])
  </code></pre>
  
  <p>Now...</p>
  
  <pre><code>&gt;&gt;&gt; df_sum.join(emp, on='TUCASEID', how='inner')
  Traceback (most recent call last):
    File "&lt;input&gt;", line 1, in &lt;module&gt;
    File "/usr/local/lib/python2.7/site-packages/pandas/core/frame.py", line 3829, in join
      rsuffix=rsuffix, sort=sort)
    File "/usr/local/lib/python2.7/site-packages/pandas/core/frame.py", line 3843, in _join_compat
      suffixes=(lsuffix, rsuffix), sort=sort)
    File "/usr/local/lib/python2.7/site-packages/pandas/tools/merge.py", line 39, in merge
      return op.get_result()
    File "/usr/local/lib/python2.7/site-packages/pandas/tools/merge.py", line 193, in get_result
      rdata.items, rsuf)
    File "/usr/local/lib/python2.7/site-packages/pandas/core/internals.py", line 3873, in items_overlap_with_suffix
      to_rename)
  ValueError: columns overlap but no suffix specified: Index([u'TUCASEID'], dtype='object')
  </code></pre>
  
  <p>Well, that's weird, the only column that appears in both data frames is the one to join over, but well, let's concur[1]:</p>
  
  <pre><code>&gt;&gt;&gt; df_sum.join(emp, on='TUCASEID', how='inner', rsuffix='r')
  Empty DataFrame
  Columns: [TUCASEID, t070101, t070102, t070103, t070104, t070105, t070199, t070201, t070299, shopping, year, TUCASEIDr, status]
  Index: []
  </code></pre>
  
  <p>Despite there being a huge intersection. What's going on here?</p>
  
  <pre><code>&gt;&gt;&gt; pd.__version__
  '0.15.0'
  </code></pre>
  
  <p>[1]: I actually enforced integer for dtype of the joining column because it said "object" there, made no difference:</p>
  
  <pre><code>&gt;&gt;&gt; emp.dtypes
  TUCASEID     int64
  status      object
  dtype: object
  &gt;&gt;&gt; df_sum.dtypes
  TUCASEID    int64
  (...)
  shopping    int64
  year        int64
  dtype: object
  </code></pre>