BUG: Remove error raise to allow same input function on the same column in named aggregation
- [x] closes #28426 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry


https://github.com/pandas-dev/pandas/pull/27921#issuecomment-521794197

this was kind of mentioned in the PR27921, and the case used in the issue is basically the same as having `min` or `max`. Previously it was not raising error is because I deliberately skipped this case since i thought this removal should be treated in different PR. Feel free to review and comment your ideas.

this solution looks tricky though, but i tested on other cases like lambda or `min`, 'max` stuff, they are all working except the partials. So I made this change (although looks like two different issues) @TomAugspurger  can you merge master @jreback @TomAugspurger  merged to master and fixed tests

since this PR was quite long ago, so i hereby put little summary to the outcome of this PR, with the PR merged:
```python
df = pd.DataFrame({"A": [1, 2]})
df.groupby([0, 0]).agg(['sum', 'sum'])
```
will still raise an error, but
```python
df = pd.DataFrame({"A": [1, 2], "B": [3, 4]})
df.groupby("A").agg(a=("B", "sum"), b=("B", "sum"))
```
will not raise because of users specifying different new column names for it
 emm, not sure what caused it. 🤔  @charlesdong1991 we had some CI issues a few weeks back; if you merge master and repush should be resolved thanks, @WillAyd merged! feel free to review and see if you think it's worth it to leave this PR open! can you merge master Hello @charlesdong1991! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-12-03 14:45:06 UTC merged! 
could you pls take a look? @jreback  this looks good @charlesdong1991 can you merge master and ping on green. ping

@jreback  ping  @jreback  thanks @charlesdong1991 