This is a bug report, I'm using the portable version of WinSCP 5.2.3 build 3436.
When I select a group of files in the windows explorer and drag and drop them into a remote folder in the WinSCP window, if these are files that are already present, I am prompted, most logically, with a dialogue asking if I want to overwrite the first file.
However, even if I choose "Yes to All", for every following file already present, I will be asked, again and again, if I want the remote file to be overwritten by the local file.
So, the "to All" option isn't taken into account.
I'm attaching a screenshot, for clarity.
Well, that's it. Thanks for the renewed efforts with that great program :)