[SPARK-12874][ML] ML StringIndexer does not protect itself from column name duplication

## 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.

mllib/src/main/scala/org/apache/spark/ml/feature/StringIndexer.scala

@@ -150,6 +150,7 @@ class StringIndexerModel (
         "Skip StringIndexerModel.")
       return dataset
     }
+    validateAndTransformSchema(dataset.schema)
 
     val indexer = udf { label: String =>
       if (labelToIndex.contains(label)) {

mllib/src/test/scala/org/apache/spark/ml/feature/StringIndexerSuite.scala

@@ -118,6 +118,17 @@ class StringIndexerSuite
     assert(indexerModel.transform(df).eq(df))
   }
 
+  test("StringIndexerModel can't overwrite output column") {
+    val df = sqlContext.createDataFrame(Seq((1, 2), (3, 4))).toDF("input", "output")
+    val indexer = new StringIndexer()
+      .setInputCol("input")
+      .setOutputCol("output")
+      .fit(df)
+    intercept[IllegalArgumentException] {
+      indexer.transform(df)
+    }
+  }
+
   test("StringIndexer read/write") {
     val t = new StringIndexer()
       .setInputCol("myInputCol")