JSFiddle - React, Tailwind, and code Playground

HTML

<html xmlns="http://www.w3.org/1999/xhtml">
    <head>
    <title></title>
    </head>
    <body>
    <input name="txtName" type="text" id="txtName" />
    <input type="submit" name="btnSave" value="Save" id="btnSave" />
    </body>
    </html>

JavaScript

$('#btnSave').click(function (e) {
    var reportRecipients = "A, c , b,";
    checkDuplicate(reportRecipients);

});


function checkDuplicate(reportRecipients) {
    var recipientsArray = reportRecipients.split(','),
        textHash = {};
    for(var i=0; i<recipientsArray.length;i++){
        var key = $.trim(recipientsArray[i].toLowerCase());
        console.log("lower:" + key);
        if(textHash[key]){
            alert("duplicated:" + key);
            return true;
        }else{
            textHash[key] = true;
        }
    }
    alert("no duplicate");
    return false;
}