DOC: documented that .apply(func) executes func twice on the first time
Related issues are #2656, #2936 and #6753.

good..thanks!
 @anton-d An advice: try to not mix tabs and spaces for indentation.

And something else, the alignment of your code example was not fully correct so it wouldn't render well (the `..ipython::` should start at the same indentation as the text of the warning), but that are tricky rst things to get right, so no problem (I corrected it here: https://github.com/pydata/pandas/commit/95a562cb01d6f0954cdc2021e02d4d8234ae5ba3). And thanks for the contribution!
 is this deployed ?
i still got the issue(first group called twice): with version=0.24.2
d = pd.DataFrame({"a":["x", "y"], "b":[1,2]})
```
def identity(df):
    print (df)
    return df

d.groupby("a").apply(identity)

```

outputs:

   a  b
0  x  1
   a  b
0  x  1
   a  b
1  y  2
