Put three duplicates in a folder, and the Finder assures you that they take three times the space of one of them, but that simply isn’t true. Where this gets a little confusing is that the Finder doesn’t tell you that the duplicate takes no extra space. No data has been copied or duplicated at all. That file, which would take several seconds at least to really copy, is duplicated instantly. Select it in the Finder and use the Duplicate command in the Finder’s contextual menu (Control-click, etc.). Find a large file, at least 10 GB in size. This can also apply to whole folders, provided that they’re copied according to these rules. In practice, these include all copies and duplicates made within the same volume by the Finder, and most made by apps.

copying must be performed using either of two specific commands (both forms of copyItem()) in the FileManager.

both the original and copy files must be on the same APFS volume, so sharing the same file system.

The conditions which have to be met for macOS to create a clone are simple: Unlike a hard link, the clone is a separate file, with its own iNode. What it does is create a clone file, which is a bit like a hard link, in that the file record points to the same data as the original. Thankfully, APFS is much smarter than that, and regularly – indeed, as a rule whenever it can – doesn’t copy any data at all. Duplicate or copy a file, and macOS copies all the data to a new storage area and makes that a different file. Most of us still think of file systems in simple terms, like HFS+.

In reality, on a modern Mac, duplicates often come free, and don’t even clutter your backups. This article explains why deduplication may now achieve little real saving of space, and why the savings it might claim are almost certainly gross overestimates. Back in the days of HFS+ I can recall housekeeping apps which have freed up more than ten percent of a hard disk in the space of a few minutes. Selling ‘housekeeping’ utilities for macOS is a lucrative trade, and one feature which we all succumb to is the promise of deduplication: finding multiple copies of files, so we can delete unnecessary ‘clutter’ and free up storage space.

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