Eclipse PDE documentation claims:
You can think of the input object as the document or file that is being edited. Changes made in an editor are not committed until the user saves them.
Only one editor can be open for any particular editor input in a workbench page. For example, if the user is editing readme.txt in the workbench, opening it again in the same perspective will activate the same editor. (You can open another editor on the same file from a different workbench window or perspective).
Obviously it is possible to open a file using different editors - for example .java file using default java editor, and then text editor (by 'open with'). Is this part of the documentation wrong? Or is IEditorInput different for these two editors? I'm just wondering.
It was like that originally, and I think their point was double-clicking on an already open file will just bring it to the front, not open a new editor with the same information. Tht's still true, but that's not the whole truth :-)
I believe that section needs to be expanded, as I'm pretty sure the capability you refer to has been there for years as well. There's a "New Editor" entry in the editor tab context menu that allows you to open the same file in the same type of editor too.