db={"users":[{user1:"a",user2:"b"},{user1:"b",user2:"a"},{user1:"c",user2:"d"},{user1:"d",user2:"c"}]}

db.users.aggregate([{$group:{_id:{$cond:[{$gt:["$user1","$user2"]},{user1:"$user1",user2:"$user2"},{user1:"$user2",user2:"$user1"}]},count:{$sum:1}}},{$match:{count:{$gt:1}}},{$project:{_id:0,user1:"$_id.user1",user2:"$_id.user2"}}])