Database
[{"_id":"1235","details":[{"productId":"PRODUCT1","myList":["0001","0001","0002","0002",]},{"productId":"PRODUCT2","myList":["0002","0002","1234","1234",]}]}]
Query
db.collection.aggregate([{$addFields:{details:{$map:{input:"$details",in:{$mergeObjects:["$$this",{myList:{$setUnion:"$$this.myList"}}]}}}}}])