BUG: Left join on index and column gives incorrect output
```python
import numpy as np
import pandas as pd

df_left = pd.DataFrame(index=["a", "b"])
df_right = pd.DataFrame({"x": ["a", "c"]})

pd.merge(df_left, df_right, left_index=True, right_on="x", how="left")
#      x
# 0.0  a
# NaN  b
```
#### Problem description

This is closely related to https://github.com/pandas-dev/pandas/issues/28220 but deals with the values of the `DataFrame` rather than the index itself.  When left joining on an index and a column it looks like the value `"b"` from the index of `df_left` is somehow getting carried over to the column `x`, but `"a"` should be the only value in this column since it's the only one that matches the index from `df_left`.  This is happening on 0.25.1 and master, and has been a bug for some time according to https://github.com/pandas-dev/pandas/issues/28220.

#### Expected Output

```python
     x
a    a
b  NaN
```

#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.3.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 18.7.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 0.25.1
numpy            : 1.16.4
pytz             : 2019.2
dateutil         : 2.8.0
pip              : 19.1.1
setuptools       : 41.0.1
Cython           : None
pytest           : 5.0.1
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : 7.7.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
</details>

Seems related to this issue: https://github.com/pandas-dev/pandas/issues/17257