Tested on Aqua Data Studio 14.0.0-beta-14 Build #: 32768 on Ubuntu 12.04( Mongo DB 2.4.3)  and Windows XP ( Mongo DB 2.0.9).

 

Next SQL gives  exception "com.mongodb.MongoException$DuplicateKey: { "serverUsed" : "localhost/127.0.0.1:27017" , "err" : "E11000 duplicate key error index: Test_Case.tmp_from_subquery.$_id_  dup key: { : ObjectId('51b1c259ecde088596e01dd7') }" , "code" : 11000 , "n" : 0 , "connectionId" : 11 , "ok" : 1.0} "

 

Seems it happens on insert into temporary collection. All works correct when UNION used instead UNION ALL