Uploading a 512x512 site icon creates a "cropped" duplicate in media library

[adamkheckler]

To reproduce:

1. Install latest stable WP (4.6.1 currently) using Twenty Sixteen theme. No plugins.
2. Find an image that's exactly 512x512 pixels.
3. In the Customizer, go to Site Identity and click Select Image in the Site Icon section.
4. Upload the 512x512 image you prepared in Step 2.
5. Your site icon is uploaded. Click Save and Publish.
6. Look in your Media Library.

Expectation: one image present, which should be the image you just uploaded at the recommended size.

Reality: your original image is there, but there's also a duplicate prepended with "cropped-".

IMAGE1

I don't understand why an image that's already 512x512 needs to be cropped. The resulting images are exactly the same dimensions and filesize.

Here's what it looks like in the uploads directory:

IMAGE2

At first glance, I thought this was a recurrence of this ticket:

https://core.trac.wordpress.org/ticket/33011

... but the filenames aren't quite the same, so I suspect it's a new bug.