After years of buying hard drives and moving files around, digital hoarders like myself eventually wind up with duplicate files all over the place. A few gigabytes here and there can add up, so Gemini aims to help you find and delete duplicate files across devices. It's a beautifully designed app, and if you're doing basic directory comparisons, is fast as lightning.
To start, you just drag a few folders onto Gemini. It'll start scanning with a big circular progress bar moving around. One of the awesome features I've not seen elsewhere is the ability to see the results of a scan and do something about them while the app continues to scour your folders. Gemini is fast even though you can be impatient, which is a plus if you scan a really large set of folders.
One issue I did run into was scanning a nearly-full 500 GB drive that had my Time Machine backups on it. The machine using Gemini only had about 5 GB of space available on its drive, and after about an hour of Gemini checking the external drive, my system locked up as it ran out of drive space! This is an oddball issue, according to MacPaw, and they are looking into it. That said, all the other scans of large folders I attempted worked great and went quickly without eating up HD space. Gemini's speed is really impressive.
As a bonus, Gemini offers a delightfully designed interface. When you are shown duplicates, you can easily see where they are located, pick the ones you want to delete, then view a receipt of what you want to remove before you delete it. Then, when you are sure, Gemini "shreds" the receipt (and files) before your eyes. Taking something rather tedious (file management) and making it fun turns Gemini into a must-have app if you have years of files stored on external drives around your home.