Here ya’ll can learn how to report dupes on the site. I reckon.
 
Oh, and I forgot to say. If you encounter images and you’re not sure what to do about them, you can always go to to the official Derpibooru IRC channel (#derpibooru on irc.ponychat.net) and ask some of our friendly ponies for advice. You can find more info on the IRC channel at the Contact link at the bottom of every page on Derpibooru.
 
 
Click on this picture to be taken to my Derpibooru upload guides. The first part is outdated; the Derpibooru search engine is much better than it was, but I haven’t gotten around to updating that part of the guide yet. But if you want to upload images to Derpibooru it and the subsequent guides should give you a good idea of what to do.