Database
[{_id:123,messages:[{user:1234,sentAt:ISODate("2022-03-01T00:00:00.000Z")},{user:1234,sentAt:ISODate("2022-03-02T00:00:00.000Z")},{user:888,sentAt:ISODate("2022-03-03T00:00:00.000Z")},]}]
Query
db.collection.aggregate([{$addFields:{messages:{$reduce:{input:"$messages",initialValue:[],in:{$cond:[{$in:["$$this.user","$$value.user"]},"$$value",{"$concatArrays":["$$value",["$$this"]]}]}}}}}])