del df[k] fails for a non-unique key
``` python
import pandas as pd
df=pd.DataFrame({0:[0,1],1:[0,1],2:[4,5]})
df.columns=["a","b","a"]
res=df.pop("a")
...

/home/user1/src/pandas/pandas/core/internals.py in _delete_from_block(self, i, item)
    971         """
    972         block = self.blocks.pop(i)
--> 973         new_left, new_right = block.split_block_at(item)
    974 
    975         if new_left is not None:

/home/user1/src/pandas/pandas/core/internals.py in split_block_at(self, item)
    195             return None, None
    196 
--> 197         if loc == 0:
    198             # at front
    199             left_block = None

ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()
```
