[{name:"XYZ",location:["A","B","C","D"]},{name:"XYZ",location:["M","N"]},{name:"ABC",location:["P","Q","R","S"]}]

db.collection.aggregate([{$match:{name:"XYZ"}},{$group:{_id:"$name",location:{$push:"$location"}}},{$project:{_id:0,name:"$_id",location:{$reduce:{input:"$location",initialValue:[],in:{$concatArrays:["$$value","$$this"]}}}}}])