### What changes were proposed in this pull request?
When `toPandas` API works on duplicate column names produced from operators like join, we see the error like:
```
ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().
```
This patch fixes the error in `toPandas` API.
### Why are the changes needed?
To make `toPandas` work on dataframe with duplicate column names.
### Does this PR introduce any user-facing change?
Yes. Previously calling `toPandas` API on a dataframe with duplicate column names will fail. After this patch, it will produce correct result.
### How was this patch tested?
Unit test.
Closes#28025 from viirya/SPARK-31186.
Authored-by: Liang-Chi Hsieh <viirya@gmail.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
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