code change based on review

pandas/core/groupby/generic.py

@@ -43,7 +43,7 @@
 
 from pandas._typing import FrameOrSeries
 import pandas.core.algorithms as algorithms
-from pandas.core.base import DataError
+from pandas.core.base import DataError, SpecificationError
 import pandas.core.common as com
 from pandas.core.frame import DataFrame
 from pandas.core.generic import ABCDataFrame, ABCSeries, NDFrame, _shared_docs
@@ -230,6 +230,13 @@ def aggregate(self, func, *args, **kwargs):
         elif func is None:
             # nicer error message
             raise TypeError("Must provide 'func' or tuples of '(column, aggfunc).")
+        elif isinstance(func, list) and len(func) > len(set(func)):
+
+            # GH 28426 will raise error if duplicated function names are used and
+            raise SpecificationError(
+                "Function names must be unique if there is no new column "
+                "names assigned"
+            )
 
         func = _maybe_mangle_lambdas(func)
 
@@ -857,6 +864,12 @@ def aggregate(self, func=None, *args, **kwargs):
             kwargs = {}
             if not columns:
                 raise TypeError(no_arg_message)
+        elif isinstance(func, list) and len(func) > len(set(func)):
+            # GH 28426 will raise error if duplicated function names are used and
+            raise SpecificationError(
+                "Function names must be unique if there is no new column "
+                "names assigned"
+            )
 
         if isinstance(func, str):
             return getattr(self, func)(*args, **kwargs)

pandas/tests/groupby/aggregate/test_aggregate.py

@@ -9,6 +9,7 @@
 
 import pandas as pd
 from pandas import DataFrame, Index, MultiIndex, Series, compat, concat
+from pandas.core.base import SpecificationError
 from pandas.core.groupby.generic import _make_unique, _maybe_mangle_lambdas
 from pandas.core.groupby.grouper import Grouping
 import pandas.util.testing as tm
@@ -349,6 +350,18 @@ def test_uint64_type_handling(dtype, how):
     tm.assert_frame_equal(result, expected, check_exact=True)
 
 
+def test_func_duplicates_raises():
+    # GH28426
+    gr = pd.Series([1, 2, 3]).groupby([0, 0, 1])
+    msg = "Function names"
+    with pytest.raises(SpecificationError, match=msg):
+        gr.agg(['sum', 'sum'])
+
+    df = pd.DataFrame({"A": [0, 0, 1, 1], "B": [1, 2, 3, 4]})
+    with pytest.raises(SpecificationError, match=msg):
+        df.groupby("A").agg(['min', 'min'])
+
+
 class TestNamedAggregationSeries:
     def test_series_named_agg(self):
         df = pd.Series([1, 2, 3, 4])