JSFiddle - React, Tailwind, and code Playground

HTML

<form>
    <input type="hidden" name="test1" value="val" />
    <input type="hidden" name="test2" value="val" />
    <input type="hidden" name="test1" value="val" />
</form>
<pre></pre>

JavaScript

$("pre").html(decodeURI(serializeWithDuplicates("form")));

function serializeWithDuplicates(form) {
    var pairs = $(form).serializeArray();
    for (var i = 0; i < pairs.length; i++)
        for (var j = i + 1; j < pairs.length; j++)
            if (pairs[i].name === pairs[j].name)
                pairs[i].isDuplicate = pairs[j].isDuplicate = true;
    for (var i = 0; i < pairs.length; i++)
	    if (pairs[i].isDuplicate)
            pairs[i].name += "[]";
    return $.param(pairs);
}