I cannot get downloaded from Safari not overwrite previous filenames.

It's not working like it should. Give this a try: boot into Safe Mode according to Start up your Mac in safe mode - Apple Support and test to see if the problem persists. Reboot normally and test again.

NOTE: Safe Mode boot can take up to nearly twice as long as a normal boot as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

It has to do with that folder's permissions. A good website that offers multiple file downloads at once should offer an FTP server to download the files at once directly with an FTP client such as Filezilla. The browser just assumes any file with the same name should not be overwritten, and asks.

FTP is called file transfer protocol. It is the old method of sharing files over TCP/IP which is the network address tool the internet is based upon.

If you are trying to obtain files from a computer in the same room, there is no reason to use FTP.

If you are trying to obtain files from a computer in the same company over the same company network, again, FTP is generally not needed, your machine just needs to be mapped to the company's network address system using SMB for Windows or Mac File Sharing for Macs.

When it is over the internet, setting up an FTP server on one end, and using FTP on your end is often easiest if the machine has its own distinct internet address.

http://www.anydesk.com can setup file sharing over the internet if the other machine is behind a router/firewall or wireless access point but still on the internet.

When you are downloading files from Safari, they are generally somewhere on the internet, but they don't have to be if the right URL address given for the file's links.

afs:// is generally given for Apple file sharing

smb:// is generally given for Windows file sharing

http:// will work, but the file generally needs to be zipped up before hosting on a web based platform in winzip, 7zip, or Zipit for the Mac.

if you are redirected to a link like http://..../filename.doc that is going to a Word Document, and doesn't always transfer right over a browser such as Safari, Chrome, Firefox, Edge, or Internet Explorer.