Database
[{name:"c1",arr:[{partyName:"p1",poNumber:"789",},{partyName:"p1",poNumber:"700",},{partyName:"p3",poNumber:"889",}]},{name:"c2",arr:[{partyName:"p1",poNumber:"789",},{partyName:"p2",poNumber:"700",},{partyName:"p3",poNumber:"889",}]}]
Query
db.collection.aggregate([{$match:{name:"c1"}},{$project:{res:{$reduce:{input:"$arr",initialValue:[],in:{$setUnion:["$$value",["$$this.partyName"]]}}}}}])