[{"key":1,"tag":["one","two"]},{"key":2,"tag":["one","two"]},{"key":3,"tag":["two","one"]},{"key":4,"tag":["two"]},{"key":5,"tag":["two"]}]

db.collection.aggregate({"$unwind":"$tag"},{"$sort":{key:1,tag:1}},{"$group":{"_id":"$key","tag":{"$push":"$tag"}}},{"$group":{"_id":"$tag","field":{"$push":"$$ROOT"}}})