De dupe in Dataframe

val df = Seq(
  ("d1", "2018-09-20 10:00:00", "blah1"),
  ("d2", "2018-09-20 09:00:00", "blah2"),
  ("d1", "2018-09-20 10:01:00", "blahnew")
).toDF("documentId","timestamp","anotherField")

import org.apache.spark.sql.functions.row_number
import org.apache.spark.sql.expressions.Window

val w = Window.partitionBy($"documentId").orderBy($"timestamp".desc)
val Resultdf = df.withColumn("rownum", row_number.over(w))
     .where($"rownum" === 1).drop("rownum")

Resultdf.show()
input:

+----------+-------------------+------------+
|documentId|          timestamp|anotherField|
+----------+-------------------+------------+
|        d1|2018-09-20 10:00:00|       blah1|
|        d2|2018-09-20 09:00:00|       blah2|
|        d1|2018-09-20 10:01:00|     blahnew|
+----------+-------------------+------------+
output:

+----------+-------------------+------------+
|documentId|          timestamp|anotherField|
+----------+-------------------+------------+
|        d2|2018-09-20 09:00:00|       blah2|
|        d1|2018-09-20 10:01:00|     blahnew|
+----------+-------------------+------------+