html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,performed_via_github_app,issue
https://github.com/pydata/xarray/issues/5361#issuecomment-846307431,https://api.github.com/repos/pydata/xarray/issues/5361,846307431,MDEyOklzc3VlQ29tbWVudDg0NjMwNzQzMQ==,5635139,2021-05-21T23:24:27Z,2021-05-21T23:24:40Z,MEMBER,"That's surprising indeed. 

I confirmed the bug was present in 0.17.0

It also seems unrelated to uniqueness in the non-grouped-by dimension:

```python
In [7]: data['y'].values = [0,1,2,3]

In [8]: data['y'].values
Out[8]: array([0, 1, 2, 3])

In [9]: data
Out[9]:
<xarray.DataArray (x: 4, z: 2)>
array([[ 0.13156972,  0.13986012],
       [ 1.61815504,  0.11421297],
       [ 0.15819393, -0.5183183 ],
       [ 0.30672251,  0.34373302]])
Coordinates:
  * x        (x) <U1 'a' 'b' 'a' 'c'
    y        (x) int64 0 1 2 3
Dimensions without coordinates: z

In [10]: data.T.groupby('x').mean()
Out[10]:
<xarray.DataArray (z: 2, x: 3)>
array([[ 0.14488182,  1.61815504,  0.30672251],
       [-0.18922909,  0.11421297,  0.34373302]])
Coordinates:
  * x        (x) object 'a' 'b' 'c'
    y        (x) int64 0 1 2 3   # <-- the size must be 3!!
Dimensions without coordinates: z
```","{""total_count"": 1, ""+1"": 1, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,898657012