pivot_table should fail if duplicate entries exist
This looks like #147, but I don't have permission to re-open that.

Consider the following situation:

```
In [3]: dt
Out[3]: 
   foo  bar  baz
0  1    2    3  
1  4    5    6  
2  1    2    4  

In [4]: pandas.pivot_table(dt, values="baz", rows=["foo"], cols=["bar"])
Out[4]: 
  bar  2    5  
foo            
1      3.5  NaN
4      NaN  6  
```

Notice the column for `foo == 1 && bar == 2` shows a value of `3.5`. Either this should be in the resulting table twice, or the command should fail with an error.
