OnChange Code Not Working With Multiple Forms

I am completely new to AJAX, so I got this code from another site and installed it for what I need. It works great if I have only one form on the page ... however, once I have multiple forms on the page, it stops working but it doesnt throw an error code, so I have no clue where to look. Any help would be appreciated!

Select Field Code:

Code:

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onChange="showField(this.value, 'QQTrim')"

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Ajax.js:

Code:

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function GetXmlHttpObject() {
        var xmlHttp=null;
        try {
                xmlHttp=new XMLHttpRequest();
        }
                catch (e) {
                        try {
                                xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
                        }
                catch (e) {
                        xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
                }
        }
        return xmlHttp;
}

function showField(str, id){
        var url="inc/ajaxforms.php"
        url=url+"?modField="+id
        url=url+"&modString1="+str
        url=url+"&sid="+Math.random()
        xmlHttp=GetXmlHttpObject();
        xmlHttp.onreadystatechange= function() {
        if (xmlHttp.readyState==4)
            if (xmlHttp.status==200)
                writeHTML(xmlHttp, id);
    }
    xmlHttp.open("GET",url,true);
    xmlHttp.send(null);       
}

function writeHTML(req, id) {
    document.getElementById(id).innerHTML = req.responseText;
}

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Have you checked to make sure they are not using the same id?