It looks like your WP Jquery Lightbox plugin is generating these extra captions.
Check out the source of the jquery.lightbox.js file:
... cut ...
var s="";
if (title != '') {
s="<span id="titleText">" + title + '</span>';
}
if (caption != '') {
if (title != ''){
s += '<br />';
}
s += '<span id="captionText">' + caption +'</span>';
}
... cut ...
where you can have both a title and a caption for each image.
The title part is picked up from the image link title, the image title or the image alt attribute:
if (this.title) { //title of link
title = this.title;
} else if (jqImg.attr('title')) {
title = jqImg.attr('title'); //grab the title from the image if the link lacks one
} else if(jqImg.attr('alt')){
title = jqImg.attr('alt'); //if neither link nor image have a title attribute
}
In your case it seems to be the image alt attribute.
If the image caption is equal to the image alt attribute, then you should only get the title part.
So to get rid of the duplications, you make sure the image alt attribute is the same as the image caption. Notice that the comparison is made after this replacement:
captionText = $.trim(captionText).replace('’', ''').replace('’', '\''); //http://nickjohnson.com/b/wordpress-apostrophe-vs-right-single-quote
if (title.toLowerCase() == captionText.toLowerCase()) {
title = caption; //to keep linked captions
caption = ''; //but not duplicate the text
}