I wrote some php code a few years back, that I used to find duplicates in the images on wallhaven and the old site (that this site is based off of), and it worked really well, and it is not taxing at all. It is really good at finding images that are the same (regardless of their uploaded resolution, even if they have been reversed/flipped or the image modified (like some images having copyright or tag line, and another image not). It doesn't consider any EXIF data or file size or resolution in the search.
I never did much with it because I had a lot going on in my life then. Honestly, it worked better than any other type of image duplicate finder that I have ever used. You just add a few more fields in your database for each image, and when images get uploaded, they get the generated search numbers, so it's live and in real time. You only have to do a full-index once. Re-indexing would not be needed unless at some point, the code was improved even more than it already is.
You may be already doing this, but it would be nice if someone uploaded a higher resolution image of a duplicate (or a image with copyright tagging or web site tagging over layed), that it would (and this is just me brain storming, and subject to feedback)...
a) accept the new image and copy over the tags of the previous lower version
b) mark it in such a way that it is a higher resolution version of a previously uploaded image
c) in some fashion or another that is intuitive and respective of the original uploader, provide a symbolic link to the new image
If your interested in the code, I will dig it up and provide some examples to show it really works. You can even compare it with your IQDB system results. In fact, I would be super curious to how it performs against it.