I have a photo blog with thousands of posts on Tumblr. When posting a photo, how can I make sure I haven't already seen it somewhere and posted it sometime?
You can now easily find duplicate photos in your Tumblr blog by using this Chrome extension:
Right now it can only detect duplicates in your posts, so you'll have to post the photo first. To keep your blog free of duplicates, you can scan weekly or monthly.
This extension is based on the open source pHash perceptual hash fingerprinting library, which allows you to detect duplicates with surprising accuracy without installing a dedicated desktop application.
Disclaimer: I am the author.
You may want to use XKit. It is a set of browser extensions for Tumblr that adds / modifies behavior of the site.
On an update on Oct 30, 2013, the developer added a feature to recognize already-re logged posts.
Added option to hide posts already reblogged / drafted / queued using One-Click Postage on the dashboard.
If you're in mobile you may want to consider XKit mobile
Please note, however, that the extension is not officially from Tumblr and that it occasionally breaks when Tumblr updates its UI/API. It is generally fixed right away but there's no guarantee that will keep being the case, since it's a side-project for a single developer.