Fixed reindexing arith with duplicates
Closes https://github.com/pandas-dev/pandas/issues/35194

Still need to run ASV on this. This was a regression from 0.25.x to 1.0. It doesn't have to go in 1.1, but probably better for 1.1.0.rc0 than a 1.1.1 release.
cc @jbrockmendel. Having trouble with ASV right now. Things seem OK in this small benchmark though

```python
In [15]: df1 = pd.DataFrame(np.random.randn(1000, 500))

In [16]: df2 = pd.DataFrame(np.random.randn(1000, 501))

In [17]: %timeit df1 + df2
# master
3.8 ms ± 115 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

# PR
3.77 ms ± 104 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
``` LGTM thanks @TomAugspurger 