[{"_id":ObjectId("5fb60945df88631311ab8a48"),"name":"x-market","tags":["water","soda","water"]},{"_id":ObjectId("5fb60945df88631311ab8a49"),"name":"y-market","tags":["juice","water"]}]

db.collection.aggregate([{$unwind:{path:"$tags",includeArrayIndex:"index"}},{$group:{_id:{_id:"$_id",tags:"$tags"},name:{$first:"$name"},index:{$push:"$index"}}},{$project:{_id:"$_id._id",name:"$name",tag:"$_id.tags",index:"$index"}}])