A post that is completely identical to its parent down to the pixel and was either posted after another post with the same Pixel Hash or has no source while their parent does, and thus is inferior for Danbooru purposes.
If a PNG is a pixel-perfect duplicate of a JPEG, lossy-lossless may also apply.
There are three ways to know if similar images are pixel-perfect duplicates:
On the upload page, a red warning with a link to the existing post will be shown under the tag box, and a red pixel-perfect duplicate label will show be shown at the bottom.
On the image's asset page, clicking on Pixel Hash will show other posts and assets with the same Pixel Hash. These are pixel-perfect duplicates.
An image's asset page can be opened by clicking on the text below the image in the uploader showing the filesize, file type and resolution or by clicking the two arrows beside the resolution on a post's page.
In 2019 Twitter stopped compressing as harshly as they used to and in late 2020 Pixiv started losslessly recompressing their JPGs to strip metadata, leading to many cases where the file sizes are different despite the images being pixel-perfect duplicates.
For uploads already posted, the post with the lowest ID should be the parent, and the rest should be tagged as pixel-perfect duplicate and parented to it. The only exception is if the post with the lowest id has no source, which then the next highest id with a valid source should be the parent.
In general, uploaders should avoid posting duplicates. For duplicates already posted, uploaders are responsible for properly tagging and parenting their duplicates to the existing post.
Do not flag posts just because they are under this tag if they come from a first-party source.
