Writing an add-in to detect duplicates

[Asheesh Laroia <asheesh asheesh org>]

I have a problem where I accidentally import photos more than once.  I'd 
like to compare photos based on some hash (SHA1 perhaps) of the decoded 
image data inside the file.  (Hashing the raw file is problematic because 
metadata may have changed, and I figure I should hash the decoded data 
baecause in the case of a lossless format it's still the same image when 
recompressed.)

It would be great to memoize this in a metadata tag in the image so that 
way every program could just calculate this for the file it's trying to 
import, see if this value exists already, and ask me what I wanted it to 
do.

I'm happy to write such a plugin/add-in/extension, but I don't know where 
to find out more about the interface for doing so.  Anyone have any 
thoughts on this plan (maybe there are better ways to detect duplicates?) 
or how to code such a thing (is there plugin writing documentation?)?

Thanks!

-- Asheesh.

--
In the stairway of life, you'd best take the elevator.