fix #39556 (infer_freq not working with freq="H" and DST
- check that the delta are unique before checking if they are day multiples
- add test with freq="H" that raises the bug

- [x] closes #39556 
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry => not sure where to enter this ...
