## What changes were proposed in this pull request?
ML StringIndexer does not protect itself from column name duplication.

We should still improve a way to validate a schema of `StringIndexer` and `StringIndexerModel`.  However, it would be great to fix at another issue.

## How was this patch tested?
unit test

Author: Yu ISHIKAWA <yuu.ishikawa@gmail.com>

Closes #11370 from yu-iskw/SPARK-12874.