[{"_id":"post1","comments":[{"user":"goodman","text":"hello","replies":[{"user":"littleboy","text":"hi"}]},{"user":"richpapa","text":"haha"},{"user":"goodman","text":"how is it going?!"}]},{"_id":"post2","comments":[{"user":"katze","text":"meow!","replies":[{"user":"kitten","text":"miaw miaw!"},{"user":"littleboy","text":"mioooooo"},{"user":"goodman","text":"meme"}]},{"user":"kitten","text":"ich bin katze","replies":[{"user":"sally","text":"ja, du bist katze maw"}]}]}]

db.collection.aggregate([{$unwind:"$comments"},{$unwind:"$comments.replies"},{$group:{_id:"allusers","user2":{$push:"$comments.user"},user1:{$push:"$comments.replies.user"}}},{$project:{users:{$concatArrays:["$user1","$user2"]}}},{$unwind:"$users"},{$group:{_id:"$users",count:{$sum:1}}},{$match:{count:{$gt:1}}}])