Question: If I had two divs, both with id="myDiv", would `$("#myDiv").fadeOut();` fade both divs out? Or would it fade only the first/second? Or none at all?

How do I change which one it fades out?

**Note:** I know duplicate id's is against standards but I'm using the fancybox modal popup and it duplicates specified content on your page for the content of the popup. If anyone knows a way around this (maybe I'm using fancybox wrong) please let me know.

Answer: 
"Note: I know duplicate id's is against standards"

Then don't do it. You have already figured out two problems. It violates standards, and it interferes with jQuery's (and indeed the regular DOM's) selection mechanism. There will probably be more issues in the future.

Quite possibly, you *are* using fancybox wrong, in which case I hope someone familiar with it helps you. Or worse, if the script itself is flawed, you shouldn't use it.
I recently had a similar problem. I had a single page that displayed different content through an Ajax load. There is a button that triggers the server to generate a PDF image.

My original solution was to use a class selector, which worked fine, but caused friction with another developer. There were so many classes already in the links that using a class as an identifier made the code ugly. So I used a name attribute:

```
<a name="printButton"....

$('a[name="printButton"]').on('click',function(){....

```

Worked just dandy!