MongooseError: Query was already executed

Had the same issues, but the release notes on mongoose helped, I chained a .clone() method to the .find() method:

https://mongoosejs.com/docs/migrating_to_6.html#duplicate-query-execution

Mongoose no longer allows executing the same query object twice. If you do, you'll get a Query was already executed error. Executing the same query instance twice is typically indicative of mixing callbacks and promises, but if you need to execute the same query twice, you can call Query#clone() to clone the query and re-execute it.

So all you need to do is to add a .clone() method to the end of the mongoose method that needs to be called concurrently like below (I restructured your code a bit):

app.get("/retrieve", function (req, res) {
    Visitor.find({}, function (err, data) {
        if (!err) {
            res.render("retrieve", { title: "View Visitor Data", records: data });
        } else {
            throw err;
        }
    }).clone().catch(function(err){ console.log(err)})
});

wrap your query within async-await.

const visitorData = async() => { 
  return await Visitor.find({});
}

Don't use the callback within the findOne, instead use .then() , after the Query, like so

 Visitor.find({})
.then(function (err, data) {
        if (!err) {
            res.render("retrieve", { title: "View Visitor Data", records: data });
        } else {
            throw err;
        }
    }

This should most likely work , otherwise use clone() if necessary , as stated by Avis

The error predominantly arises because the same query is executed more than once, the most often cause being unknowingly using promise and callback at the same time, to avoid such mistakes this error is thrown. To get rid of this error, stick to either promise or callback when executing this query method.
Use:

await Model.findOneAndUpdate(query,doc,options)
// (or) regular .then() .catch() syntax
Model.findOneAndUpdate(query)
.then(res=>...)
.catch(err=>...)

(OR)

Model.findOneAndUpdate(query, doc, options, callback)

The same principle applies to similar methods like findByIdAndUpdate.

Mongoose v6 does not allow duplicate queries.

Mongoose no longer allows executing the same query object twice. If you do, you'll get a Query was already executed error. Executing the same query instance twice is typically indicative of mixing callbacks and promises, but if you need to execute the same query twice, you can call Query#clone() to clone the query and re-execute it. See gh-7398

Duplicate Query Execution

In my case, I assumed my Apollo Server resolver will wait for the promise to resolve and return the result, but that wasn't the case. So I had to add async/await to the mongoose query.

Mongoose no longer allows executing the same query object twice. If you do, you'll get a Query was already executed error. Executing the same query instance twice is typically indicative of mixing callbacks and promises, but if you need to execute the same query twice, you can call ".clone();" to clone the query and re-execute it.

I'm not sure about the details but mine and my co-workers problems got fixed by downgrading mongoose to 5.13.14

// Mongoose 6.6.3
// Query.prototype.clone()
// Returns:
// «Query» copy
// Make a copy of this query so you can re-execute it.

// Example:`enter code here`
const q = Book.findOne({ title: 'Casino Royale' });
await q.exec();
await q.exec(); 
// Throws an error because you can't execute a query 
twice

await q.clone().exec(); // Works

My mistake was using find() for finding a record like this:

foo.find(a=> a.id === id) //It is wrong!

It should be:

foo.findById(id)

The newer version of Mongoose (version 6+) doesn't support executing the same query object twice. So, just downgrade to an older version, like version 5.13.15.

Uninstall the current Mongoose package & then download the older version:

npm uninstall mongoose

npm i mongoose@5

Hope, this can resolve the issue.