This is the error message I get when I try to open 2 documents with the same name, but in different directories, in Microsoft Excel:

A document with the name '' is already open. You cannot open two documents with the same name, even if the documents are in different folders.
To open the second document, either close the document that's currently open, or rename one of the documents.

Emphasis mine. Why can't I? We'll never know how Excel is implemented (unless there's a MS Office source code leak), but this is pretty dumb if you ask me, considering how easy it is to get by this technical difficulty. Perhaps there is some perfectly rational explanation why there is absolutely no way that Excel would be able to work with 2 documents of the same name, or if there is, it is more work than it's worth. I'd like to hear that explanation.

Oh wait, I believe I should Google it. Hmm... Toby Allen reports the same thing, and Joel Spolsky has this plausible explanation:

"I think it's because when you have an external reference (of the form foo.xls!A1) they only had 8 bytes in the data structure to store the file name. It's kind of shocking that 10 years later nobody has fixed this, ...

Well, still not fixed in Office 2003.