<input type="text" class="zfc">

 <script>
        var list;
        document.querySelector('.zfc').addEventListener('keydown', () => {
        
            var tx = String(document.querySelector('.zfc').value).toLocaleLowerCase().split(' ')
            list = [];
            tx.forEach(es => {
                let iz = 0;
                tx.forEach(el => {

                    if (es == el && el!='' && el!=' ') {
                       if (iz==0) {
                           iz++
                       }else{
                        list.push(es)
                       }
                        
                        return false;
                    }
                });
            });
            console.log(list)
            if (list.length==0) {
                document.querySelector('.zfc').style.color='green'
            }else{
              
                document.querySelector('.zfc').style.color='red'
            }
        })
    </script>