This issue occurs when editors upload 2 or more multimedia components that have the same filename. Sometimes this is a legitimate scenario, however, editors often work in silos and upload the exact same asset into the system multiple times, hence adding redundant data to the CM.
For example: Editor John is responsible for creating pages A and B, while Editor Nick is responsible for C and D. Pages A and C have the same image on them therefore they should re-use the same multimedia component; and but John and Nick are working in silos and simply upload the same asset from their Website Assets FTP location into individual MM components. John is the first one to publish his page (page A) and all is good for him. Now Nick tried to publish his page, Page C, which fails with the this Duplicate Binary error. In this case,
Forcing Unique Binary Filenames TBB is not the correct answer because it only facilitates redundant data to be created in the CMS. The correct answer here is to educate the editors so that only one of them owns uploading and naming of the assets into Tridion, while the rest use those assets.
Sometimes this issue occurs when a MM component is published along with a regular component, and after publishing gets deleted from the CM, and then another person uploads a MM component with the original name. So now the CM is out of sync with the CD and there is no way in the CM to see which items used this deleted component. In this situation the fix is described here: https://stackoverflow.com/a/14856967/1284894