How to prevent users from uploading the same file (image) multiple times?
In pseudo/quick code, this is how I would approach it.  In a custom module...


Create a table in the db, with two columns, fid and md5.
Each time you create a file, check if the md5() of whatever you consider unique is in that db, eg:

a.  select fid from TABLE where md5 = ab3783ea6c83bcd9383eef778

b.  if fid then oops, there already appears to be file blah blah blah

c.  otherwise
Store that file's fid and md5() in that table.


You could put all this logic in the validation hook when you are creating the image content.  You could even set up roles that are able to override it, etc.

Now, to the md5()ing of things.


Filename is easily changed and I don't think it is really sent to drupal anyway--but instead crafted there after the fact--so do you really want to use it?
Content of the file might be a better approach.


So, md5 could be populated with a quick md5_file(PATHTOFILE).

Now, let's say you wanted to have an option of doing this system wide or per user, eg, you don't want one user uploading multiple copies of one image or you want to have only one exact image uploaded systemwide.

Just add another column uid to the table above and check it as well, eg:

select fid from TABLE where md5 = ... and uid = $user->uid

Anyway, these are just my quick thoughts after the morning's first espresso here in D6land.  Maybe D7 has some of this already built into it and I am once again reinventing that wheel :)