For all the people who end up here, because they would like to open two Excel files with the same name at the same time:
Even though Excel itself does not permit to do so due to (certainly questionable) circumstances stated by Baldrick in his answer, there at least exist workarounds which allow to open multiple xls/xlsx files with the same name at the same time in separate Excel instances/processes.
The workarounds are explained in this thread on the How-To Geek forums.
There is even a sort of "built-in" fix with help of the setting Ignore other applications that use Dynamic Data Exchange (DDE)
, which works for me, but leads to errors when closing Excel and then trying to open a file again by double-clicking on it.
I had to to go for the registry fix, which works fine. NOTE THOUGH that this workaround, once applied, will prevent cross-referencing cells between ALL opened Excel tables (also those with different names), since the separate Excel instances are not aware of each other (at least according to tests I just made).
You may instead want to choose the fix which adds a new context menu item Open Separate
to the Explorer and only use it if you in fact want to open two files with the same name at the same time.