# Buggy code
def _gotitem(self, key, ndim, subset=None):
    """
    sub-classes to define
    return a sliced object

    Parameters
    ----------
    key : string / list of selections
    ndim : 1,2
        requested ndim of result
    subset : object, default None
        subset to act on
    """
    # create a new object to prevent aliasing
    if subset is None:
        subset = self.obj

    # we need to make a shallow copy of ourselves
    # with the same groupby
    kwargs = {attr: getattr(self, attr) for attr in self._attributes}
    self = self.__class__(subset, groupby=self._groupby[key], parent=self, **kwargs)
    self._reset_cache()
    if subset.ndim == 2:
        if is_scalar(key) and key in subset or is_list_like(key):
            self._selection = key
    return self


# Traceback (Exception)
In [13]: df = pd.DataFrame({"A": pd.to_datetime(['2015', '2017']), "B": [1, 1]})

In [14]: df
Out[14]:
A  B
0 2015-01-01  1
1 2017-01-01  1

In [15]: df.set_index("A").groupby([0, 0]).resample("AS")
Out[15]: DatetimeIndexResamplerGroupby [freq=<YearBegin: month=1>, axis=0, closed=left, label=left, convention=e, base=0]

In [16]: df.set_index("A").groupby([0, 0]).resample("AS").agg(['sum', 'count'])
---------------------------------------------------------------------------
Exception                                 Traceback (most recent call last)
<ipython-input-16-5f1c18a8d4ac> in <module>()
----> 1 df.set_index("A").groupby([0, 0]).resample("AS").agg(['sum', 'count'])

~/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/resample.py in aggregate(self, arg, *args, **kwargs)
339
340         self._set_binner()
--> 341         result, how = self._aggregate(arg, *args, **kwargs)
342         if result is None:
343             result = self._groupby_and_aggregate(arg,

~/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/base.py in _aggregate(self, arg, *args, **kwargs)
538             return self._aggregate_multiple_funcs(arg,
539                                                   _level=_level,
--> 540                                                   _axis=_axis), None
541         else:
542             result = None

~/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/base.py in _aggregate_multiple_funcs(self, arg, _level, _axis)
583                 try:
584                     colg = self._gotitem(col, ndim=1, subset=obj[col])
--> 585                     results.append(colg.aggregate(arg))
586                     keys.append(col)
587                 except (TypeError, DataError):

~/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/resample.py in aggregate(self, arg, *args, **kwargs)
339
340         self._set_binner()
--> 341         result, how = self._aggregate(arg, *args, **kwargs)
342         if result is None:
343             result = self._groupby_and_aggregate(arg,

~/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/base.py in _aggregate(self, arg, *args, **kwargs)
538             return self._aggregate_multiple_funcs(arg,
539                                                   _level=_level,
--> 540                                                   _axis=_axis), None
541         else:
542             result = None

~/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/base.py in _aggregate_multiple_funcs(self, arg, _level, _axis)
582             for col in obj:
583                 try:
--> 584                     colg = self._gotitem(col, ndim=1, subset=obj[col])
585                     results.append(colg.aggregate(arg))
586                     keys.append(col)

~/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/base.py in _gotitem(self, key, ndim, subset)
675                        for attr in self._attributes])
676         self = self.__class__(subset,
--> 677                               groupby=self._groupby[key],
678                               parent=self,
679                               **kwargs)

~/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/base.py in __getitem__(self, key)
241         if self._selection is not None:
242             raise Exception('Column(s) {selection} already selected'
--> 243                             .format(selection=self._selection))
244
245         if isinstance(key, (list, tuple, ABCSeries, ABCIndexClass,

Exception: Column(s) B already selected

# Fixed code
def _gotitem(self, key, ndim, subset=None):
    """
    sub-classes to define
    return a sliced object

    Parameters
    ----------
    key : string / list of selections
    ndim : 1,2
        requested ndim of result
    subset : object, default None
        subset to act on
    """
    # create a new object to prevent aliasing
    if subset is None:
        subset = self.obj

    # we need to make a shallow copy of ourselves
    # with the same groupby
    kwargs = {attr: getattr(self, attr) for attr in self._attributes}

    # Try to select from a DataFrame, falling back to a Series
    try:
        groupby = self._groupby[key]
    except IndexError:
        groupby = self._groupby

    self = self.__class__(subset, groupby=groupby, parent=self, **kwargs)
    self._reset_cache()
    if subset.ndim == 2:
        if is_scalar(key) and key in subset or is_list_like(key):
            self._selection = key
    return self