If nothing else, perhaps this belated reply will remind me when I inevitably forgot how to avoid the “Replace File” warning error:
Using Photoshop 7 on Windows, to create a batch capable action, it is important NOT to record while opening the sample file. Doing so apparently loads that sample files name into the Action, and every time you try to run that action in batch mode, even with the ‘override open’ option checked in automate tasks, photoshop remembers that one file name and tries to overright it with each Save for Web. If you do have an OPEN FILE item in your action, select it and delete it and the error messages will go away.