How to check whether a user exists before posting a data through axios.post method

Question

I am creating a simple app using vue.js to control mail delivery and i would like to achieve the following: Before i send a data through axios post method coming from the v-model i need to check first,whether the apartment already exists and if it exists I will just add the information of the mail otherwise, i will post the whole data including the name, id, apartment and the mail field info that happens to be an object.

Here is my json.server data

{
"blocoA": [
{
  "name": "michele ",
  "apartament": "100",
  "mail": [
    {
      "code": "3445566775677",
      "quantity": 1,
      "date_recieved":"31/01/2022"

    }
  ],
  "id": 1
},
{
  "name": "becky",
  "apartament": "10",
  "mail": [
    {  
      "code":"6789900099876655"
      "quantity": 2
      "date_recieved":"25/01/2022"
    }
  ],
  "id": 2
}

],

This is my axios method:

  const baseURL = "http://localhost:3000/blocoA";
   async addMail() {
  try {
    //const res =
     await axios.post(baseURL, 
    { name: this.name,
      apartament: this.apart,
      mail: [{
        code:this.code
        quantity:this.mail.quant
        date_received:this.date
        }]
     }).then(()=>{
       this.isSuccess = true
     });

ok, the help i need is when the addMail() function is called i would like to check if the apartment already exists in the database and if it does then i will add up only the infomations of the mail field objects otherwise i will post all the data. I want to avoid duplicating apartment when they already exist.

The desired result :

         {
           "blocoA": [
                 {
                  "name": "michele ",
                  "apartament": "100",
                  "mail": [
                          {
                           "code": "3445566775677",
                           "quantity": 1,
                           "date_recieved":"10/01/2022"

                           },
                           {
                            "code": "99999999999999999",
                            "quantity": 1,
                            "date_recieved":"13/01/2022"

                            }
                           ],
                     "id": 1
                 },

NB: Now I have a list of objects in the mail field and didn't duplicate the apartment for the same Tenant and avoided having different IDs too.