You can use the following parameter, during the initialization:

        'once_per_user':    true

If you would like to display it once for users who already signed up, then add:

        'once_per_filled':    true

The initialization will look like:

<script type='text/javascript'>                                                                                        
jQuery(document).ready(function() {
    jQuery('body').sspopup({'once_per_user':true});
});
</script> 

Of course, you can add many more additional paramaters, see the full list in the Documentation in the downloaded ZIP file.

In the most cases, it caused by a duplicated jQuery file. If your website already use jQuery, you don’t need to include it again, because it will be included twice. Check your website and look for jquery.js or jquery[version number].js, then change the source to use only one.

It is also very important to include the jQuery file BEFORE you add the plugin file, so it has to be:

<script type="text/javascript" src="js/jquery.js"></script> //first: jQuery file
<script type="text/javascript" src="js/ssp.min.js"></script> //second: Plugin file
<script type='text/javascript'> //third: Initialization                                                      
jQuery(document).ready(function() {
    jQuery('body').sspopup();
});
</script>