Database
[{"_id":0,country:"GB"},{"_id":1,country:"FR"},{"_id":2,country:"DE"},{"_id":3,country:"DE"},{"_id":4,country:"DE"},{"_id":5,country:"BR"}]
Query
db.collection.aggregate([{$match:{country:{$in:["GB","FR","DE"]}}},{$group:{_id:"$country",data:{$first:"$$ROOT"}}},{$replaceRoot:{newRoot:"$data"}}])