Because of the unique way that AEM stores assets, if you have the exact same file in two locations in the repository, physically only 1 file is stored. The reason being, that each file has a unique SHA1 checksum, the checksum value is used to create a folder/file in the datastore[1]. In this way, you don't duplicate storage for the same file. If the file is modified in any way and uploaded again, only then will a new file in the datastore be created.
However, having a central DAM is always preferable as all instances of an asset will get updated at the same time, otherwise you have to update all nodes that pointed to the original file, which can become inefficient and error prone.
Regards,
Opkar
[1]https://docs.adobe.com/docs/en/cq/5-6-1/core/administering/persistence_managers.html