BUG: DatetimeIndex constructed with Timestamps on DST border are converted to the same Timestamp
```
In [16]: pd.DatetimeIndex([pd.Timestamp('2016-10-30 03:00:00+0300', tz='Europe/Helsinki'), pd.Timestamp('2016-10-30 03:00:00+0200', tz='Europe/Helsinki')])
Out[16]: DatetimeIndex(['2016-10-30 03:00:00+02:00', '2016-10-30 03:00:00+02:00'], dtype='datetime64[ns, Europe/Helsinki]', freq=None)
```

For Helsinki, DST "falls back" on `2016-10-30` and therefore`03:00:00` occurs twice with 2 different UTC offsets (`+0300` and `+0200`). When constructing the `DatetimeIndex` above, the UTC offset of the first argument is incorrectly converted to the offset after the DST transition.

Expected:
```
Out[16]: DatetimeIndex(['2016-10-30 03:00:00+03:00', '2016-10-30 03:00:00+02:00'], dtype='datetime64[ns, Europe/Helsinki]', freq=None)
```
