read_html repeats table (regression in 0.23)
I'm running into an issue extracting a dataframe from a HTML page when using pandas 0.23. Before my environment was updated, I was using pandas 0.22 and the issue did not occur. If I install a new environment with 0.22, the issue does not occur, so I think this is probably a regression. I've cached the relevant HTML at [`trailheads.html.txt`](https://github.com/pandas-dev/pandas/files/2242597/trailheads.html.txt).

Here is the code:

```python
import requests
import pandas
url = 'https://github.com/pandas-dev/pandas/files/2242597/trailheads.html.txt'
response = requests.get(url)
wide_df, = pandas.read_html(
    response.text,
    header=1,
    attrs = {'id': 'cs_idLayout2'},
    flavor='html5lib',
    parse_dates=['Date'],
)
wide_df = wide_df.iloc[:, :6]
```

Below I'm included a subset of the resulting rows of the table to demonstrate what I mean by repeat. It seems the table has been repeated a single time leading to double the number of rows plus one (since the header is repeated as a row). Below I've excluded all rows besides the top and bottom 2 for brevity.

| Date | Happy Isles->Little Yosemite Valley | Happy Isles->Sunrise/Merced Lake (pass through) | Glacier Point->Little Yosemite Valley | Sunrise Lakes | Lyell Canyon |
|------------|-------------------------------------|-------------------------------------------------|---------------------------------------|---------------|--------------|
| 7/28/2018 | 0 | 0 | 0 | 0 | 0 |
| 7/29/2018 | 0 | 0 | 0 | 0 | 0 |
| 12/17/2018 | 18 | 6 | 6 | 9 | 15 |
| 12/18/2018 | 18 | 6 | 6 | 9 | 15 |
| Date | Happy Isles->Little Yosemite Valley | Happy Isles->Sunrise/Merced Lake (pass through) | Glacier Point->Little Yosemite Valley | Sunrise Lakes | Lyell Canyon |
| 7/28/2018 | 0 | 0 | 0 | 0 | 0 |
| 7/29/2018 | 0 | 0 | 0 | 0 | 0 |
| 12/17/2018 | 18 | 6 | 6 | 9 | 15 |
| 12/18/2018 | 18 | 6 | 6 | 9 | 15 |
