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This is up and running.

"Similar on moe" and "similar on danbooru" are replaced with "Find dupes" and "Find similar". These both go to the image search. "Find similar" searches all known databases on the image search. "Find dupes" searches only moe.

Uploaded posts run through the image search; matches over 90% bring up the dupe check page, which gives instructions to delete the post, reparent the old one to the new one, or reparent and add the "duplicate" tag.

The edit drop-down works for images on moe; using it on external images should simply do nothing.

Direct links in the search results are provided for moe images, but images on other boards just go to the post page.

The image search accepts a url parameter, like the main image search, for scripting. It accepts some others related to this; I'll make a separate post about that when I post the updated Greasemonkey script. It also outputs XML, if asked.