I create a file in a share.
I create a file with the same name on a desktop.
When I try to copy file from desktop to a share, I get:
Cannot copy file: cannot create a file when that file already exists.
It works from Windows XP.
It is fixed in http://us4.samba.org/samba/history/samba-3.0.25c.html :
* Fix file overwrites from Windows 9x clients.