Just like the eD2k network, Overnet knows when files are different even if they have the same file size and name, ensuring that unique files are properly presented and that when several identical files are found it can download from those independent sources at the same time.
This also allows you to create links to the files.
However, Overnet was a closed protocol. This means that it's specifications are unknown and it's designers aren't willing to show them to the public.
The Overnet website was shut down 2006 after a RIAA lawsuit and shows a RIAA banner since. There have been no more updates and Overnet is not in use anymore.
NOTE: aMule is a separate project and is not related to Overnet in any way.