how to remove duplicate array elements in JavaScript and Vue

I am attempting to set up a Vue demo that prevents a user from inputting duplicate items in an array. The inputs values with the following text field and button:

 <div>
    <input v-model="numberValue" type="number" />
 </div>
 <div>
   <button @click="submit" type="submit">Submit</button>
</div>

After pushing numberValue.value into the numArray array, I then for to loop through the items in numArray. Then, I used the indexOf method in a conditional to check for occurances of array items in newArray. If array items don't already exist in newArray, then they will be pushed into newArray.

const submit = () => {
  numArray.value.push(numberValue.value)
  
  newArray.value = [];
  
  for (let i = 0; i < numArray.value.length; i++) { 
    if(newArray.value.indexOf(numArray.value[i]) === -1) { 
        newArray.value.push(numArray.value[i]); 
        console.log(newArray.value)
    } else if (newArray.value.indexOf(numArray.value[i]) !== -1) {
      window.alert('No Duplicates Allowed!')
    }
  }
}

I then attempted to set up an else if statement that targets instances when the user tries to input a duplicate array item. On the else if, I added a window.alert to alert the user when entering a duplicate. After entering a duplicate, I see the alert. I then see the alert on all subsequent inputs, even ones that are not duplicates. How can I go about properly setting up the else if statement to target duplicates and trigger an alert?

You can easily achieve this requirement by comparing the input value with indexOf and lastIndexOf values in a numArray. And to have non-duplicate values you can use Set.

Working Demo :

new Vue({
  el: '#app',
  data: {
    numberValue: null,
    numArray: []
  },
  methods: {
    submit() {
      this.numArray.push(this.numberValue);
      if (this.numArray.indexOf(this.numberValue) !== this.numArray.lastIndexOf(this.numberValue)) {
        alert('No Duplicates Allowed!');
      }
      this.numArray = [...new Set(this.numArray)]
    }
  }
})

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.5.17/vue.js"></script>
<div id="app">
  <input v-model="numberValue" type="number" />
  <button @click="submit" type="submit">Submit</button>
  <br><br>
  {{ numArray }}
</div>