firebase onSnapshot how to refresh arrays to avoid duplicate keys?

here is the code im using to listen to changes in my firestore database:

 async mounted() {
let id = [];
let orders = [];
await db.collection("orders").onSnapshot(doc => {
  includeMetadataChanges: true;

  doc.docs.forEach(x => {
    id.push(x.id);
    let z = Object.assign(x.data(), { id: x.id });
    orders.push(z);
  });
});

I'm using vuejs, adding this listener on the mount stage so the arrays depending on this snapshot keep refreshing. but I'm facing a problem which is when changes happen to the database my new snapshot adds the data to the array which results in duplicate keys all over, I can't find an efficient way to reset the arrays on each snapshot before inserting the new version.

id array is an array I use to extract the id then insert it inside the orders array so I can use it internally.

Edit:

 async mounted() {
let id = [];
let orders = [];
await db.collection("orders").onSnapshot(doc => {
  includeMetadataChanges: true;
orders = []
id = []
  doc.docs.forEach(x => {
    id.push(x.id);
    let z = Object.assign(x.data(), { id: x.id });
    orders.push(z);
  });
}); 
  
console.log(orders)

when I reset orders array in callback I get an empty array.

Edit 2- i found the error: I miscalculated where to place the save the array.

  this.$store.dispatch("mystore/saveOrders", orders);

I should have placed it inside the onSnapshot function so each time it runs I do the save, at first I had in the mounted function right after the onsnapshot listener but I had to reset orders as mentioned in the answer by Frank van Puffelen.