Reactjs – Why does I get a duplication error everytime I post a new data

sorry if the title doesnt fit to the problem.

My question is how do I implement a post method with data and obeject? at first submit, i can create an appointment but when I resubmit the data from insomnia, i get an error message

{
    "success": false,
    "message": "E11000 duplicate key error collection: MCityVet.appointments index: patientId_1 dup key: { patientId: null }",
    "statusCode": 500
}

unless if i delete to the database, i can create new appointment but still if i resubmit, i get an error.

heres my appointment model:

import mongoose from 'mongoose';

const appointmentSchema = new mongoose.Schema({
    schedule: {
        type: String,
    },
    technicianName: {
        type: mongoose.Schema.Types.ObjectId,
        ref: 'User',
    },
    firstName: {
        type: String,
    },
    lastName: {
        type: String,
    },
    phone: {
        type: String,
    },
    patient: {
        typeOfAnimal: {
            type: String,
        },
        numberOfHeads: {
            type: Number,
        },
        age: {
            type: Number,
        },
        services: [{
            type: String,
        }],
        address: {
            type: String,
        },
        landmark: {
            type: String,
        },
    },
});

const Appointment = mongoose.model('Appointment', appointmentSchema);
export default Appointment;

heres my cotroller (backend/appointment/create) :

// Create Appointment
export const createAppointment = async (req, res, next) => {
  try {
    const { schedule, technicianName, firstName, lastName, phone, patient } = req.body;

    // Create a new appointment instance
    const newAppointment = new Appointment({
      schedule,
      technicianName,
      firstName,
      lastName,
      phone,
      patient,
    });

    // Save the appointment to the database
    const savedAppointment = await newAppointment.save();

    res.status(201).json(savedAppointment);
  } catch (error) {
    next(error);
  }
};

i also tried the uuid but still got a duplication/null error. heres a dummy json data:

{
  "schedule": "2023-11-15",
  "technicianName": "653f74dee45212eac760a17d",
  "firstName": "John",
  "lastName": "Doe",
  "phone": "123-456-7890",
  "patient": {
    "typeOfAnimal": "Cat",
    "numberOfHeads": 2,
    "age": 2,
    "services": ["Grooming"],
    "address": "1234 Elm",
    "landmark": "Near park"
  }
}