[SPARK-21144][SQL] Print a warning if the data schema and partition schema...

[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 <yamamuro@apache.org>

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