Felt like announcing it!
The template appears as below:
The template should be used on the lower quality of two identical (or virtually identical) images. In the case of images identical in every way, I'd personally pick the better file name, but theoretically, either could be tagged in that case.
I'll be going through to replace some of the standard deletion templates I've placed on some file pages with this if this is the one that fits, and later (probably after my bot is approved/flagged), I'll be going through and finding more duplicates and tagging them as such.
(The template's page can be found at Template:Duplicate.)