Merge duplicates

[albertogomcas]

Occasionally PDFs get added more than once in a library, this is easy to detect later with the find duplicates tool.

However the problem is that deleting the duplicates may end up creating problems if somebody has used the static link of the deleted entry somewhere else (internal wiki, word documents...). If the link was nicely formatted (which by the way leads me to #144 (comment)) you can search in the DB and probably find again the document, but if the link was the default one (something like "http:domain/librarian/stable.php?id=1360" well, good luck with that.

It would great to have the ability to "merge" duplicates, meaning one of the PDFs can be deleted, but the duplicated entries (librarian IDs and permanent links) all redirect to the remaining one.

[Zugrik]

My 2 cents here, that it also should do a thing, if deleted item is in some projects, has some notes and/or discussions.