While working on our Zencart modules which make use of jquery and its plugins extensively, I often run into this issue: one module which is released long ago is set to use 1.4.2 version while another module which has just been released is set to use 1.4.4 (or even 1.5.0). This results in multiple versions of jquery being loaded on the same page and slows down the site (or even stops certain things from working correctly).
To resolve that problem, I have added a new feature to our loader, this feature will greatly help in reducing the risk of loading the same library more than once. If you are a developer, feel free to grab the latest version from github and give it a try, you will find lots of sample code inside.
We will continue working on this feature to refine it.