[SPARK-17432][SQL] PreprocessDDL should respect case sensitivity when checking duplicated columns

## What changes were proposed in this pull request?

In `PreprocessDDL` we will check if table columns are duplicated. However, this checking ignores case sensitivity config(it's always case-sensitive) and lead to different result between `HiveExternalCatalog` and `InMemoryCatalog`. `HiveExternalCatalog` will throw exception because hive metastore is always case-nonsensitive, and `InMemoryCatalog` is fine.

This PR fixes it.

## How was this patch tested?

a new test in DDLSuite

Author: Wenchen Fan <wenchen@databricks.com>

Closes #14994 from cloud-fan/check-dup.