[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.

1 files changed, 7 insertions, 0 deletions

diff --git a/sql/core/src/test/scala/org/apache/spark/sql/execution/command/DDLSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/execution/command/DDLSuite.scala
index fd35c987ca..05f826a11b 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/execution/command/DDLSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/execution/command/DDLSuite.scala
@@ -371,6 +371,13 @@ class DDLSuite extends QueryTest with SharedSQLContext with BeforeAndAfterEach {
       sql("CREATE TABLE tbl(a int, a string) USING json")
     }
     assert(e.message == "Found duplicate column(s) in table definition of `tbl`: a")
+
+    withSQLConf(SQLConf.CASE_SENSITIVE.key -> "false") {
+      val e2 = intercept[AnalysisException] {
+        sql("CREATE TABLE tbl(a int, A string) USING json")
+      }
+      assert(e2.message == "Found duplicate column(s) in table definition of `tbl`: a")
+    }
   }
 
   test("create table - partition column names not in table definition") {