resample becomes non-deterministic, depending on DateTimeIndex values
#### Minimal Example
```python
import datetime as dt
import numpy as np
import pandas as pd


def np_to_df(data, start_time):
    index = pd.DatetimeIndex(
        [start_time + dt.timedelta(milliseconds=t) for t in range(len(data))])
    df = pd.DataFrame(data, index=index)
    return df


# generate sample data
data = np.sin(np.arange(1000) / 30)

# create DataFrames with DateTimeIndices
df_1 = np_to_df(data, dt.datetime(2019, 9, 30, 9, 41))
df_2 = np_to_df(data, dt.datetime(2019, 9, 30, 9, 42))

# print difference before resampling
print("error_1-2:", np.mean(np.abs(df_1.values - df_2.values)))

# resample
df_1 = df_1.resample("19ms").mean()
df_2 = df_2.resample("19ms").mean()

# print difference after resampling
print("error_1-2:", np.mean(np.abs(df_1.values - df_2.values)))
```

Output:
```
error_1-2: 0.0
error_1-2: 0.04119868246404099
```
#### Problem description
When you give the exact same data to the resample function, it becomes non-deterministic if the `DateTimeIndex` has differing values - even though the frequency is the same.

#### Expected Output
The values of the two `DataFrames` should be exactly the same.

#### Output of ``pd.show_versions()``
<details>

- commit           : None
- python           : 3.6.8.final.0
- python-bits      : 64
- OS               : Linux
- OS-release       : 4.15.0-51-generic
- machine          : x86_64
- processor        : x86_64
- byteorder        : little
- LC_ALL           : None
- LANG             : C.UTF-8
- LOCALE           : en_US.UTF-8
- pandas           : 0.25.1
- numpy            : 1.17.2
- pytz             : 2019.2
- dateutil         : 2.8.0
- pip              : 9.0.1
- setuptools       : 41.0.1
- Cython           : None
- pytest           : 4.4.0
- hypothesis       : None
- sphinx           : None
- blosc            : None
- feather          : None
- xlsxwriter       : None
- lxml.etree       : 4.3.3
- html5lib         : 0.999999999
- pymysql          : None
- psycopg2         : 2.7.7 (dt dec pq3 ext lo64)
- jinja2           : 2.10.1
- IPython          : 7.1.1
- pandas_datareader: None
- bs4              : None
- bottleneck       : None
- fastparquet      : None
- gcsfs            : None
- lxml.etree       : 4.3.3
- matplotlib       : 3.1.1
- numexpr          : None
- odfpy            : None
- openpyxl         : None
- pandas_gbq       : None
- pyarrow          : None
- pytables         : None
- s3fs             : None
- scipy            : 1.3.1
- sqlalchemy       : 1.3.7
- tables           : None
- xarray           : None
- xlrd             : None
- xlwt             : None
- xlsxwriter       : None

</details>

Happy about any help, @jreback ?