In [1]: import dask.dataframe as dd

In [2]: import pandas as pd

In [3]: df = pd.DataFrame({'a': [1,1,2], 'b': [4,5,6]})

In [4]: df.groupby('a').agg(c=('b', 'mean'), d=('b', 'mean'))
Out[4]: 
     c    d
a          
1  4.5  4.5
2  6.0  6.0

In [5]: dd.from_pandas(df).groupby('a').agg(c=('b', 'mean'), d=('b', 'mean'))
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[5], line 1
----> 1 dd.from_pandas(df).groupby('a').agg(c=('b', 'mean'), d=('b', 'mean'))

File ~/polars-api-compat-dev/.venv/lib/python3.12/site-packages/dask_expr/_groupby.py:1955, in GroupBy.agg(self, *args, **kwargs)
   1954 def agg(self, *args, **kwargs):
-> 1955     return self.aggregate(*args, **kwargs)

File ~/polars-api-compat-dev/.venv/lib/python3.12/site-packages/dask_expr/_groupby.py:1934, in GroupBy.aggregate(self, arg, split_every, split_out, shuffle_method, **kwargs)
   1931 if arg == "size":
   1932     return self.size()
-> 1934 result = new_collection(
   1935     GroupbyAggregation(
   1936         self.obj.expr,
   1937         arg,
   1938         self.observed,
   1939         self.dropna,
   1940         split_every,
   1941         split_out,
   1942         self.sort,
   1943         shuffle_method,
   1944         self._slice,
   1945         *self.by,
   1946     )
   1947 )
   1948 if relabeling and result is not None:
   1949     if order is not None:

File ~/polars-api-compat-dev/.venv/lib/python3.12/site-packages/dask_expr/_collection.py:4835, in new_collection(expr)
   4833 def new_collection(expr):
   4834     """Create new collection from an expr"""
-> 4835     meta = expr._meta
   4836     expr._name  # Ensure backend is imported
   4837     return get_collection_type(meta)(expr)

File ~/.local/share/uv/python/cpython-3.12.6-linux-x86_64-gnu/lib/python3.12/functools.py:993, in cached_property.__get__(self, instance, owner)
    991 val = cache.get(self.attrname, _NOT_FOUND)
    992 if val is _NOT_FOUND:
--> 993     val = self.func(instance)
    994     try:
    995         cache[self.attrname] = val

File ~/polars-api-compat-dev/.venv/lib/python3.12/site-packages/dask_expr/_groupby.py:439, in GroupbyAggregation._meta(self)
    437 @functools.cached_property
    438 def _meta(self):
--> 439     return self._lower()._meta

File ~/.local/share/uv/python/cpython-3.12.6-linux-x86_64-gnu/lib/python3.12/functools.py:993, in cached_property.__get__(self, instance, owner)
    991 val = cache.get(self.attrname, _NOT_FOUND)
    992 if val is _NOT_FOUND:
--> 993     val = self.func(instance)
    994     try:
    995         cache[self.attrname] = val

File ~/polars-api-compat-dev/.venv/lib/python3.12/site-packages/dask_expr/_reductions.py:440, in ApplyConcatApply._meta(self)
    438 @functools.cached_property
    439 def _meta(self):
--> 440     meta = self._meta_chunk
    441     aggregate = self.aggregate or (lambda x: x)
    442     if self.combine:

File ~/.local/share/uv/python/cpython-3.12.6-linux-x86_64-gnu/lib/python3.12/functools.py:993, in cached_property.__get__(self, instance, owner)
    991 val = cache.get(self.attrname, _NOT_FOUND)
    992 if val is _NOT_FOUND:
--> 993     val = self.func(instance)
    994     try:
    995         cache[self.attrname] = val

File ~/polars-api-compat-dev/.venv/lib/python3.12/site-packages/dask_expr/_groupby.py:213, in GroupByApplyConcatApply._meta_chunk(self)
    210 @functools.cached_property
    211 def _meta_chunk(self):
    212     meta = meta_nonempty(self.frame._meta)
--> 213     return self.chunk(meta, *self._by_meta, **self.chunk_kwargs)

File ~/polars-api-compat-dev/.venv/lib/python3.12/site-packages/dask_expr/_groupby.py:530, in DecomposableGroupbyAggregation.chunk_kwargs(self)
    527 @property
    528 def chunk_kwargs(self) -> dict:
    529     return {
--> 530         "funcs": self.agg_args["chunk_funcs"],
    531         "sort": self.sort,
    532         **_as_dict("observed", self.observed),
    533         **_as_dict("dropna", self.dropna),
    534     }

File ~/.local/share/uv/python/cpython-3.12.6-linux-x86_64-gnu/lib/python3.12/functools.py:993, in cached_property.__get__(self, instance, owner)
    991 val = cache.get(self.attrname, _NOT_FOUND)
    992 if val is _NOT_FOUND:
--> 993     val = self.func(instance)
    994     try:
    995         cache[self.attrname] = val

File ~/polars-api-compat-dev/.venv/lib/python3.12/site-packages/dask_expr/_groupby.py:411, in GroupbyAggregationBase.agg_args(self)
    408 @functools.cached_property
    409 def agg_args(self):
    410     keys = ["chunk_funcs", "aggregate_funcs", "finalizers"]
--> 411     return dict(zip(keys, _build_agg_args(self.spec)))

File ~/polars-api-compat-dev/.venv/lib/python3.12/site-packages/dask/dataframe/groupby.py:875, in _build_agg_args(spec)
    873 for funcs in by_name.values():
    874     if len(funcs) != 1:
--> 875         raise ValueError(f"conflicting aggregation functions: {funcs}")
    877 chunks = {}
    878 aggs = {}

ValueError: conflicting aggregation functions: [('mean', 'b'), ('mean', 'b')]