GET not returning sent message. Only inbox items

You can use below aggregation to make sure that there will be only one document returned for every (recipient, sender) pair:

db.Messages.aggregate([    {        $addFields: { conversants: [ "$recipientId", "$creator" ] }    },    {        $match: { conversants: req.query.recipientId }    },    {        $addFields: { conversant: { $arrayElemAt: [ { $filter: { input: "$conversants", cond: { $ne: [ "$$this", "5df0014e25ee451beccf588a" ] } }  } , 0 ] } }    },    {        $sort: { creationDate: 1 }    },    {        $group: {            _id: "$conversant",            message: { $first: "$message" },            recipientId: { $first: "$recipientId" },            creator: { $first: "$creator" },            messageTrackingId: { $first: "$messageTrackingId" },            creationDate: { $first: "$creationDate" }        }    },            {        $lookup: {            from: "users",            let: { creator: "$creator" },            pipeline: [                { $match: { $expr: { $eq: [ "$_id", "$$creator" ] } } },                { $project: { creatorName: 1 } }            ],            as: "creatorName"        }    },    {        $addFields: { creatorName: { $arrayElemAt: [ "$creatorName", 0 ] } }    }])

The idea here is that you create additional field which represents two ids: creator and recipient. This will allow you to do two things: filter using recipientId (who may also be the sender) - step two, and select conversant value which is always the second person - no matter if a person specified in your request sent or received a message in that conversation. Then you can $group on that field to make sure that you'll get only single message in every "conversation". Instead of using $addToSet with $arrayElemAt you can just run $first. Relatively heavy $lookup can also be run as a last step since you need to get that data once per "conversant".

EDIT: first three stages can be replaced with below stages - that should significantly improve performance since the filtering will be applied as soon as possible:

{    $match: {        $or: [ { recipientId: req.query.recipientId  }, { creator: req.query.recipientId  } ]    }},{    $addFields: {         conversant: {             $cond: [ { $ne: [ "$recipientId", req.query.recipientId  ] }, "$recipientId", "$creator" ]         }     }},