ERR: DataFrame.fillna with non-unique mapping
- [x] I have checked that this issue has not already been reported.
- [x] I have confirmed this bug exists on the latest version of pandas.
- [x] (optional) I have confirmed this bug exists on the master branch of pandas.

---

```python
import numpy as np
import pandas as pd

df = pd.DataFrame({"a": [1.0, np.nan]})
s = pd.Series([0.0, 2.0], index=["a", "a"])
df.fillna(s)  # works silently on master or 1.1.2
#      a
# 0  1.0
# 1  0.0
```

#### Problem description

DataFrame.fillna using a Series with a duplicated index is potentially ambiguous, but the filling works silently (apparently using the first value).

#### Expected Output

Maybe a warning that the operation is ambiguous?