Ensuring, each image is unique

Hello! I maintain a collection of pictures by the same author. The images come from several different sources and sometimes overlap (different names, same file). How can I make sure, each file is only uploaded once? Is there a plugin, perhaps, that keeps a checksum of each image in the database and checks each new image's checksum -- rejecting duplicates?

Gallery version (not just "2"): 2.3.1
PHP version (e.g. 5.1.6): 5.2.10
Webserver (e.g. Apache 1.3.33): 2.2.14
Database (e.g. MySql 5.0.32): 5.0.45
Operating system (e.g. Linux): FreeBSD