Hyperbolic Software has debuted Umbrella (https://www.hyperbolicsoftware.com/Umbrella.html), a duplicate prevention utility for macOS. 

Once a source is added, it monitors the folder content and reports any duplicates the moment they’re created. Duplicates are automatically marked for fast removal.

Umbrella requires macOS 10.14 or higher. It costs $19.99. A demo is available for download.