Storing emails on Network Drives as msg files - appending file name

Storing emails on Network Drives as msg files - appending file name with incremental numbers

Hi, not sure if I should post this question in Office forum or Windows, but since using Windows Explorer, figure I try here first.
For years I save my emails directly to my home drive, to avoid the use of pst files. When drag and drop the emails any subject with the same subject windows would add a incremental number to the file name, i.e. Re: Lunch Meeting (1), Re: Lunch Meeting (2).
I've only seen this occur when select a group of messages to drag and drop, never seen it add the incremental number on individual messages.
Now have a client who is insisting it worked before, that when adds an email to existing folders if it had the same subject it would ask, copy, replace or keep both. I know keep both occurs if using Offline files and synchronization and resolving conflicts. But never seen that on a basic drag and drop on an individual file without first renaming the subject or rename the file name so don't get the prompt to replace the file.
Is there any setting or registry key to force the option for client to get the keep both option.
Thanks