See the button below. For some reason RENAMEWHENSAME seems not to work. There is replace dialog displayed when a file with the same name as a copied file already exists in the destination.
I'd also remove the FILE={allfile$} since the Copy command (and most others) will work on all selected files by default and it's quicker & better to avoid using {allfiles$} unless you have to (since it can generate a really long command line).