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 ...

I have added the whatsnew entry There is some check that fails after having added the what's new entry. Any clue why? @sdementen The CI failures are due to #39688. They should be gone if you merge master. cc @mroeschke i think this is an hour-based analogue of the problem of freq=Day vs freq=DayDST.

i.e. this fixes one problem but will introduce others.  im hesitant to do that, but the long-term fix has been stuck in limbo for a while > @sdementen The CI failures are due to #39688. They should be gone if you merge master.

Do I need to do this myself? Or will this automatically be solved once someone merge my PR into master? > > @sdementen The CI failures are due to #39688. They should be gone if you merge master.
> 
> Do I need to do this myself? Or will this automatically be solved once someone merge my PR into master?

you wanna merge master yourself yeah I haven't used `infer_freq` much, but if `infer_freq` = "infer the wall time frequency" then yes this the same issue as freq=Day vs freq=DayDST. merged with master, all tests pass, ready to merge ;-) @arw2019 @mroeschke , do I need to do something more re this PR for it to be merged ? > can u also add an explicit test that is similar to the OP

The test `test_infer_freq_tz_transition` is similar to my OP in the issue. The bug was not revealing itself because the frequency "H" was not used (only "3H") and it is this frequency "H" that I uses in my OP. cc @mroeschke if any comments, merge when good Thanks @sdementen 