[BUG] `df.groupby(['a']).agg({'b': ['mean', 'mean']})` only produces a single column (as opposed to 2)

**Describe the bug**
A clear and concise description of what the bug is.

**Steps/Code to reproduce bug**

```
df = cudf.DataFrame({'a': [1,1,2], 'b': [4,5,6]})
df.groupby(['a']).agg({'b': ['mean', 'mean']})
```
outputs
```
b
mean
a
1 4.5
2 6.0
```

**Expected behavior**

what pandas does:
```python
b
mean mean
a
1 4.5 4.5
2 6.0 6.0
```

Spotted in [Narwhals](https://github.com/narwhals-dev/narwhals)

**Environment overview (please complete the following information)**
- Environment location: [Bare-metal, Docker, Cloud(specify cloud provider)]
- Method of cuDF install: [conda, Docker, or from source]
- If method of install is [Docker], provide `docker pull` & `docker run` commands used

24.10.01, google colab

**Environment details**
Please run and paste the output of the `cudf/print_env.sh` script here, to gather any other relevant environment details

**Additional context**
Add any other context about the problem here.