// I was wondering if it is possible to prevent duplicate uploads of files before they are loaded on the server?
yeah absolutely it is possible , you need to just remove the duplicates before uploading.
May be this can be useful to you :)
TRY a tool advanced system optimizer , use its duplicate file remover to remove all the duplicates .,
It really works , check it out from http://download.cnet.com/Advanced-System-Optimizer/3000-2094_4-10147659.html?tag=mncol
Select the location where you want to search the duplicate files and remove by them ..
It wll work ,