One of the webapps I'm working in is made up of many partial HTML files. If the partial requires a JavaScript library such as YUI, the YUI library is included in the partial. 

When the partials are combined at runtime, the resulting HTML often includes the YUI library several times.

```
<html>
... 
<script type="text/javascript" src="/js/yahoo/yahoo-min.js"></script>
... 
<script type="text/javascript" src="/js/yahoo/yahoo-min.js"></script>
... 
<script type="text/javascript" src="/js/yahoo/yahoo-min.js"></script>
...
</html>

```

I've seen strange behavior from including jQuery several times, especially when using AJAX. Why, specifically, is including the same JavaScript library more than once a bad idea? Why does it only sometimes cause problems?