You will need a UDF for this. Klout has a bunch of opensource HivUDFS under the package brickhouse. Here is the github link. They have a bunch of UDF's that exactly serves your purpose. Download,build and add the JAR. Here is an example
CREATE TEMPORARY FUNCTION combine AS 'brickhouse.udf.collect.CombineUDF';
CREATE TEMPORARY FUNCTION combine_unique AS 'brickhouse.udf.collect.CombineUniqueUDAF';
select combine_unique(combine(array('a','b','c'), array('b','c','d'))) from reqtable;
OK
["d","b","c","a"]