There are three blocked users in the script, sapphie (runs the Ponybooru importer, so everything here should already be imported from the other boorus) and two other users who requested to be filtered as they upload here first. I should note that these filters (with the exception of sapphie) are probably less necessary these days as the importer has dupe detection now.
[bq]Not every image from the other boorus is on this site.[/bq]
It's possible that some images were imported from other boorus first, and sometimes it can be just difficult to locate some images. I know every once in a while I come across an image that will not come up with a reverse image search
, and takes extensive looking through tags to locate.