BUG: resample with dup index broken
```
In [2]: df = DataFrame(randn(4,12),index=[2000,2000,2000,2000],columns=[ pd.Period(year=2000,month=i+1,freq='M') for i in range(12) ])

In [3]: df
Out[3]: 
       2000-01   2000-02   2000-03   2000-04   2000-05   2000-06   2000-07   2000-08   2000-09   2000-10   2000-11   2000-12
2000 -0.630327  0.698388 -0.678858 -1.289394  1.541255  1.828321 -0.157585 -0.362442  1.315526  0.783812  1.195390 -0.350228
2000 -0.213631 -0.759442 -0.418679  0.199200  0.449716 -1.936404 -1.040448 -1.867548  1.066477 -0.303183  1.982605  0.138027
2000  1.052005  1.291127  1.311332 -0.215721 -1.075875  0.209229 -0.159184 -0.662904 -1.812327  0.830689 -1.720358  1.323978
2000 -1.266226 -0.508003 -0.382924 -0.165822 -0.876964 -0.767572 -0.334778 -1.576188  0.378332 -0.617412 -1.053356  1.843076

In [4]: df.resample('Q',axis=1)
AttributeError: 'DataFrame' object has no attribute 'flags'
```
