Hey,
TL;DR
There are these two posts:
https://e621.net/post/show/1275911/
https://e621.net/post/show/1148479/
They are identical except, one of them is bigger in size (not resolution).
I simply don't know which one to delete, so I created this topic.
(My account isn't week old yet, so I cannot comment or send PMs to uploaders)

Inkbunny further compresses PNGs through their site which results in a different MD5. This has no impact on the appearance of the image, only the resulting filesize.

Something to note would be the list of hashes on submission pages.

The page in question: https://inkbunny.net/submissionview.php?id=1303890

Below the post information, you will see a section starting with something like "MD5 Hash for Page 1". Click "Show" and you will see the list of hashes for each view type, including the initial hash of the image before any site-side compression took place. The Initial hash is the same as the one for the now-deleted e6 post while the Full Size one is the same as the one posted several months ago.

The only thing that differed in this case is the image source, as some sites handle images differently.

Thank you for your respond, now I'll know where and what to look for if I run into a dilemma like this