Mongodb – Prevent duplicated user id using a locked thread in Next.js

I use MongoDB to store user data. The user id goes incrementally, such as 1, 2, 3, 4 etc when new user register.

I have the following code to generate the user id. "users" is the name of the collection where I store the user data.

// generate new user id
let uid;
const collections = await db.listCollections().toArray();
const collectionNames = collections.map(collection => collection.name);

if(collectionNames.indexOf("users") == -1){
    uid = 1;
}
else{
    const newest_user = await db.collection("users").find({}).sort({"_id":-1}).limit(1).toArray();
    uid = newest_user[0]["_id"] + 1;
}
user._id = uid;


// add and save user
db.collection("users").insertOne(user).catch((error)=>{
    throw error;
});

One concern I have now is that when two users make a request to register at the same time, they will get same maximum user id, and create the same new user id. One way to prevent it is using a locked thread. But, I think Node.js and Next.js doesn’t support multi-thread.
What are some alternatives I have to solve this problem?

In addition, _id will be the field for uid. Will it make a difference since _id can’t be duplicated.