No deduplication is done (to be efficient) when unioning RDDs/DataFrames in Spark 2.1.0+.

1. Union 2 RDDs.

   df1.union(df2)
   // or for old-fashioned RDD
   rdd1.union(rdd_2)
   

2. Union multiple RDDs.

   df = spark.union([df1, df2, df3]) // spark is a SparkSession object
   // or for old-fashioned RDD
   rdd = sc.union([rdd1, rdd2, rdd3]) // sc is a SparkContext object
   

References

Union DataFrames in Spark

Union DataFrames in Spark