JSFiddle - React, Tailwind, and code Playground

by Good Muyis

HTML

<select id="sel_1">
  <option value="one">one</option>
  <option value="two">two</option>
  <option value="three">three</option>
  <option value="four">four</option>
</select><br>

<select id="sel_2">
  <option value="one">one</option>
  <option value="two">two</option>
  <option value="three">three</option>
  <option value="four">four</option>
</select><br>

<select id="sel_3">
  <option value="one">one</option>
  <option value="two">two</option>
  <option value="three">three</option>
  <option value="four">four</option>
</select><br>

<select id="sel_4">
  <option value="one">one</option>
  <option value="two">two</option>
  <option value="three">three</option>
  <option value="four">four</option>
</select>
<br><br><br>
<input type="button" value="save" onclick="save()">

JavaScript

function save() {
    var selects = document.getElementsByTagName('select');
    var values = [];
    for(i=0;i<selects.length;i++) {
        var select = selects[i];
        if(values.indexOf(select.value)>-1) {
            alert('duplicate exists');
        }
        else 
            values.push(select.value);
            
    }
}