### What changes were proposed in this pull request?
This PR is adding support duplicated column names for `toPandas` with Arrow execution.
### Why are the changes needed?
When we execute `toPandas()` with Arrow execution, it fails if the column names have duplicates.
```py
>>> spark.sql("select 1 v, 1 v").toPandas()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/path/to/lib/python3.7/site-packages/pyspark/sql/dataframe.py", line 2132, in toPandas
pdf = table.to_pandas()
File "pyarrow/array.pxi", line 441, in pyarrow.lib._PandasConvertible.to_pandas
File "pyarrow/table.pxi", line 1367, in pyarrow.lib.Table._to_pandas
File "/path/to/lib/python3.7/site-packages/pyarrow/pandas_compat.py", line 653, in table_to_blockmanager
columns = _deserialize_column_index(table, all_columns, column_indexes)
File "/path/to/lib/python3.7/site-packages/pyarrow/pandas_compat.py", line 704, in _deserialize_column_index
columns = _flatten_single_level_multiindex(columns)
File "/path/to/lib/python3.7/site-packages/pyarrow/pandas_compat.py", line 937, in _flatten_single_level_multiindex
raise ValueError('Found non-unique column index')
ValueError: Found non-unique column index
```
### Does this PR introduce any user-facing change?
Yes, previously we will face an error above, but after this PR, we will see the result:
```py
>>> spark.sql("select 1 v, 1 v").toPandas()
v v
0 1 1
```
### How was this patch tested?
Added and modified related tests.
Closes#28210 from ueshin/issues/SPARK-31441/to_pandas.
Authored-by: Takuya UESHIN <ueshin@databricks.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
87be3641eb