BUG: DataFrame.combine with non-unique columns

```python
# Based on test_combine_datetlike_udf

df = pd.DataFrame({"A": range(5), "B": range(5)})
df.columns = ["A", "A"]

other = df.copy()
df.iloc[1, :] = None

def combiner(a, b):
    return b

>>> df.combine(other, combiner)
AttributeError: 'DataFrame' object has no attribute 'dtype'. Did you mean: 'dtypes'?
```
