ChineseImmigrants said:
So, just to be certain, the policy is to go off of visual differences, not filesize? I'm thinking about things like post #1253565 where the filesize of the tumblr version is larger than the previously uploaded (inkbunny full-res PNG) version, but there is no visual difference, even when compared using something like resemble.js.

Is the policy that if they are both the same resolution and both are (natively) PNG images, the old upload will stay, even if the filesizes are different?

Inkbunny only strips non-visual metadata on uploads to the best of my knowledge, regardless of format. For the sake of archival, it's debatable whether metadata has any use, but for loading and viewing images, it's preferable to a post without additional non-visual file size bloat.

In the past janitors/admins have deleted visually identical reuploads from non-Inkbunny sites (SoFurry, Weasyl, and Tumblr all don't do the metadata stripping), so I'd imagine that policy is still standard today.