[SPARK-21144][SQL] Print a warning if the data schema and partition schema have the duplicate columns

## What changes were proposed in this pull request?
The current master outputs unexpected results when the data schema and partition schema have the duplicate columns:
```
withTempPath { dir =>
  val basePath = dir.getCanonicalPath
  spark.range(0, 3).toDF("foo").write.parquet(new Path(basePath, "foo=1").toString)
  spark.range(0, 3).toDF("foo").write.parquet(new Path(basePath, "foo=a").toString)
  spark.read.parquet(basePath).show()
}

+---+
|foo|
+---+
|  1|
|  1|
|  a|
|  a|
|  1|
|  a|
+---+
```
This patch added code to print a warning when the duplication found.

## How was this patch tested?
Manually checked.

Author: Takeshi Yamamuro <[email protected]>

Closes #18375 from maropu/SPARK-21144-3.

Author: maropu

sql/catalyst/src/main/scala/org/apache/spark/sql/util/SchemaUtils.scala

@@ -0,0 +1,53 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.util
+
+import org.apache.spark.internal.Logging
+
+
+/**
+ * Utils for handling schemas.
+ *
+ * TODO: Merge this file with [[org.apache.spark.ml.util.SchemaUtils]].
+ */
+private[spark] object SchemaUtils extends Logging {
+
+  /**
+   * Checks if input column names have duplicate identifiers. Prints a warning message if
+   * the duplication exists.
+   *
+   * @param columnNames column names to check
+   * @param colType column type name, used in a warning message
+   * @param caseSensitiveAnalysis whether duplication checks should be case sensitive or not
+   */
+  def checkColumnNameDuplication(
+      columnNames: Seq[String], colType: String, caseSensitiveAnalysis: Boolean): Unit = {
+    val names = if (caseSensitiveAnalysis) {
+      columnNames
+    } else {
+      columnNames.map(_.toLowerCase)
+    }
+    if (names.distinct.length != names.length) {
+      val duplicateColumns = names.groupBy(identity).collect {
+        case (x, ys) if ys.length > 1 => s"`$x`"
+      }
+      logWarning(s"Found duplicate column(s) $colType: ${duplicateColumns.mkString(", ")}. " +
+        "You might need to assign different column names.")
+    }
+  }
+}

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/DataSource.scala

@@ -40,6 +40,7 @@ import org.apache.spark.sql.execution.streaming._
 import org.apache.spark.sql.sources._
 import org.apache.spark.sql.streaming.OutputMode
 import org.apache.spark.sql.types.{CalendarIntervalType, StructType}
+import org.apache.spark.sql.util.SchemaUtils
 import org.apache.spark.util.Utils
 
 /**
@@ -182,6 +183,11 @@ case class DataSource(
       throw new AnalysisException(
         s"Unable to infer schema for $format. It must be specified manually.")
     }
+
+    SchemaUtils.checkColumnNameDuplication(
+      (dataSchema ++ partitionSchema).map(_.name), "in the data schema and the partition schema",
+      sparkSession.sessionState.conf.caseSensitiveAnalysis)
+
     (dataSchema, partitionSchema)
   }