Query regarding temporarily-uploaded files

Hi all.

I have a requirement, wherein I need to allow vanilla uploads of files
to a HTTPD server.

Any client can upload any number of files (one at a time).
Also, there is just one directory, where the files get stored
"finally" (that is, after being copied from the temporary location,
via "move_uploaded_file")

Also, I have been able to get the simple file uploading running via
PHP, by picking up one of the numerous "Hello World" examples
available :)



Now, I am facing the following use-case ::

1)
User 1 starts uploading a large file, say "big_file.avi".

2)
Meanwhile, user 2 also starts uploading a (different) file, but with
the same name "big_file.avi".


In an ideal scenario, user 2 should be prompted with a message, that a
file of the same name is already being uploaded by someone else
somewhere.
Is there a way to do this?

( Note that, had the user 2 started uploading AFTER user 1 had
finished with the upload, we could probably modify the PHP-script at
the sever-side, to let user-2 know that a file of the same name
already exits. But I am failing to find a solution, when the user 2
starts the upload WHILE the large file of user 1 is in the process of
completing uploading).


Any way the issue may be solved?

I will be grateful for any pointers :)



Regards,
Ajay