BUG: positional getitem indexing with list on Series with duplicate integer index fails
#### Problem description

With a Series with integer index, `__getitem__` / [] does positional indexing, but when you have a duplicate index and use a list indexer, you get a "ValueError: cannot reindex from a duplicate axis" (with `.loc` the same works OK)

#### Code Sample, a copy-pastable example if possible

```python
In [28]: s = pd.Series(range(3), index=[1, 2, 3])

In [29]: s[1]
Out[29]: 0

In [30]: s[[1]]
Out[30]: 
1    0
dtype: int64

# duplicate index
In [31]: s = pd.Series(range(3), index=[1, 1, 3])

In [32]: s[1]
Out[32]: 
1    0
1    1
dtype: int64

In [33]: s[[1]]
...
ValueError: cannot reindex from a duplicate axis

In [34]: s.loc[[1]]
Out[34]: 
1    0
1    1
dtype: int64
```

It seems to only fail for integer index (eg with a string index `s = pd.Series(range(3), index=['a', 'a', 'b'])` the equivalent works ok)