Unable to drop na with duplicate columns

scala> val left = Seq(("1", null), ("3", "4")).toDF("col1", "col2")
left: org.apache.spark.sql.DataFrame = [col1: string, col2: string]

scala> val right = Seq(("1", "2"), ("3", null)).toDF("col1", "col2")
right: org.apache.spark.sql.DataFrame = [col1: string, col2: string]

scala> val df = left.join(right, Seq("col1"))
df: org.apache.spark.sql.DataFrame = [col1: string, col2: string ... 1 more field]

scala> df.show
+----+----+----+
|col1|col2|col2|
+----+----+----+
|   1|null|   2|
|   3|   4|null|
+----+----+----+


scala> df.na.drop("any")
org.apache.spark.sql.AnalysisException: Reference 'col2' is ambiguous, could be: col2, col2.;
  at org.apache.spark.sql.catalyst.expressions.package$AttributeSeq.resolve(package.scala:240)