Color me a little confused about the business model of free. I understand that Google gives away free email and other free software. Google tracks what you do and sells the data to make money from advertising. You are part of the product, not a customer.
Mac, iPhone and iPad users also get plenty of free software from Apple. Safari, Mail, Calendar, Reminders, iTunes, Pictures, iMovie among many. But you buy a piece of hardware for the privilege of using free software. That said, if you have duplicate photos on Mac, you can find them for free.
With Excuses to Joss Whedons Serenity TV Series and Frank Costanza's Serenity Now! episode in Seinfeld, there is little silence in the tens of thousands of pictures in the Mac's Photos app where a few thousand are duplicates. I know we're not worried about storing as much as we did for generations earlier, but who among the faithful Macs and iPhones has no more than a fair share of duplicate photos?
Enter Photos Duplicate Cleaner for Mac; a free app that does what you want to do ̵
Photos Duplicate Cleaner minimizes the tedious effort used to search the entire system for duplicate images. Instead of manually searching the Mac for duplicates, select a particular folder and let this cleaner do its work. As a result, you'll save plenty of space on your Mac. Photos Duplicate Cleaner creates batches of duplicate images so it's easy to find out how many instances of similar images are and select selected duplicates. All copies of an image are deleted except the original.
What's not to like? Well, read on.
The first thing you see in Photos Duplicate Cleaner is a request to upgrade to the Pro version, which has more features and a price tag.
 It's okay because it's still good value for free, but at least we can see where the business model is, right?
How long it takes for Photos Duplicate Cleaner to scan through your image collection depends on how many photos you have stored and how fast your Mac is, but it didn't take long to chew through 25,000 photos on my Mac – in a minute . The odometer pressure may vary.
The scan found a few hundred duplicates and showed them in a few dozen groups. Click on the ones you do not want and click on the Recycle Bin checkbox. Photos Duplicate Cleaner will delete these images from your collection.
It's all about it. Photo Duplicate Cleaner can sort through the Photos Library, or a folder full of images, even one on an external disk drive connected to the Mac. To date, the app gets a 4.5-star rating on the Mac App Store, so downloading and installation cannot be more painless.
The Pro Level version is called Duplicates Fixer Pro, and I found it at 99 cents in the Mac App Store over the weekend. Either way, you end up with few pictures, no duplicates, and a cleaner Photos library.