How can I check if my DataFrame's DatetimeIndex contains duplicate timestamps, which is a common cause of this `ValueError`?

You can check for duplicate timestamps in your DataFrame's index using a few simple methods. First, ensure your index is indeed a `DatetimeIndex`. Then, you can use `df.index.has_duplicates` which returns a boolean. To see the actual duplicate labels, you can use `df.index[df.index.duplicated()]`. This will return a DatetimeIndex containing only the duplicate timestamps. For a count, `df.index.duplicated().sum()` is useful.