[{"review":"Some review",rating:4,tour:1,user:"John",},{"review":"Some review",rating:4,tour:1,user:"John",},{"review":"Some review",rating:4,tour:1,user:"John",},{"review":"Some review",rating:4,tour:1,user:"Jenny",}]

db.collection.aggregate([{$match:{user:"John"}},{$group:{_id:{u:"$user",t:"$tour"},cnt:{$sum:1}}},{$match:{cnt:{$gt:1}}},{$project:{_id:0,user:"$_id.u",tour:"$_id.t",number_of_reviews:"$cnt"}}])