JSFiddle - React, Tailwind, and code Playground

HTML

<select id="user_ld_0_weapon_magz0" style="font:11px arial,sans-serif;"><option value="999"></option><option value="0">Hatchet mag</option><option value="1">Hunting Knife mag</option><option value="2">Flashlight mag</option><option value="3">Flashlight (Military) mag</option><option value="4">Matchbox mag</option><option value="5">Toolbox mag</option><option value="6"  selected="">Map mag</option><option value="7">Compass mag</option><option value="8">GPS mag</option></select>

<select id="user_ld_0_weapon_magz1" style="font:11px arial,sans-serif;"><option value="999"></option><option value="0">Hatchet mag</option><option value="1">Hunting Knife mag</option><option value="2" >Flashlight mag</option><option value="3">Flashlight (Military) mag</option><option value="4">Matchbox mag</option><option value="5">Toolbox mag</option><option value="6" selected="">Map mag</option><option value="7">Compass mag</option><option value="8">GPS mag</option></select>

<select id="user_ld_0_weapon_magz2" style="font:11px arial,sans-serif;"><option value="999"></option><option value="0" selected>Hatchet mag</option><option value="1">Hunting Knife mag</option><option value="2">Flashlight mag</option><option value="3">Flashlight (Military) mag</option><option value="4">Matchbox mag</option><option value="5">Toolbox mag</option><option value="6">Map mag</option><option value="7">Compass mag</option><option value="8">GPS mag</option></select>

<select id="user_ld_0_weapon_magz3" style="font:11px arial,sans-serif;"><option value="999" selected=""></option><option value="0">Hatchet mag</option><option value="1">Hunting Knife mag</option><option value="2">Flashlight mag</option><option value="3">Flashlight (Military) mag</option><option value="4">Matchbox mag</option><option value="5">Toolbox mag</option><option value="6">Map mag</option><option value="7">Compass mag</option><option value="8">GPS mag</option></select>

JavaScript

proverka = function(id) {
    var currentval,currentvals,trevoga,flag,numerok,loadout;
    var perem = '';
    flag = false;
    for(i=0;i<4;i++) {
        currentval = document.getElementById('user_ld_'+id+'_weapon_magz' + i).value;
        if(perem == '') {
            perem = currentval;
        } else {
            perem = perem + ',' + currentval;
        };
        
        for(s=0;s<4;s++) {
            currentvals = document.getElementById('user_ld_'+id+'_weapon_magz' + s).value;
            if(s != i && currentval != 999 && currentvals != 999 && currentval == currentvals) {
                currentval = 999;
                //document.getElementById('user_ld_0_weapon_magz' + i).getElementsByTagName('option')[0].setAttribute('selected','selected');
                
                flag = true;
                numerok = parseInt(document.getElementById('user_ld_'+id+'_weapon_magz'+i).value);
                trevoga = 'Duplicated tool "'+document.getElementById('user_ld_'+id+'_weapon_magz'+i).getElementsByTagName('option')[numerok+1].text + '" detected ';
            }
        }
    }
    if(flag) {
        if(id==0) { trevoga = trevoga + 'in Default loadout';};
         alert(trevoga);  
    };
}
proverka(0);