Node.js E11000 duplicate key error collection - node.js

Question

I'm trying to create a reservation with node.js and express and mongoose!
This is my Schema:
let mongoose = require("mongoose");
let Schema = mongoose.Schema;
let reservationSchema = new Schema({
car: [
{
type: Schema.Types.ObjectId,
ref: "cars",
required: true,
unique: false,
},
],
client: [
{
type: Schema.Types.ObjectId,
ref: "users",
required: true,
unique: false,
},
],
request: {
type: "Date",
default: Date.now(),
},
from: {
type: "Date",
required: true,
},
days: {
type: "Number",
required: true,
},
});
let reservation = mongoose.model("Reservation", reservationSchema);
module.exports = reservation;
And this is my function that I'm using to make it work while the dates does not match
function check(reservations, newReservation) {
console.log("chegou");
return new Promise(function (resolve, reject) {
ReservationModel.find({}, function (err, reservations) {
console.log("chegou aqui lol");
console.log([reservations]);
console.log(newReservation);
if (err) reject(err);
reservations.forEach(myFunction);
function myFunction(reservations) {
console.log("Siga ze");
console.log(reservations.car);
console.log(newReservation.car);
if ((reservations.car = newReservation.car)) {
console.log("chegou ao if");
let date = reservations.from;
let days = reservations.days;
var result = new Date(date);
result.setDate(result.getDate() + days);
console.log(result);
console.log(newReservation.from);
if (newReservation.from >= result) {
console.log("devia funfar crlh");
resolve(newReservation);
} else {
console.log("Nao funfou");
}
} else {
console.log("fds nao da esta merda pqp");
}
}
});
});
}
It reaches the
if (newReservation.from >= result) {
console.log("devia funfar crlh");
resolve(newReservation);
So it should work, but when I get to the part when it is going to save to the database it gives me the error:
E11000 duplicate key error collection: rent-a-car.reservations index: car_1 dup key: { car: ObjectId('625af24b81b1edc90fdef50a') }
Saying that the car is already in use, and I have specifically in the Schema to not be unique.
What can I do?