For whatever reason, Google Drive considers the file name to just be one of the properties of the file.
Files (v2 has a worse explanation of this strangeness, so I show v3)
The name of the file. This is not necessarily unique within a folder.
Their web interface will happily let you make such duplicates as well.
Google Drive also prefers (and threatens to demand) that applications have access only to their data.
Enhancing security controls for Google Drive third-party apps (but so far they haven’t enforced policy)
https://duplicati-oauth-handler.appspot.com/
will give Duplicati access to all your files, if that’s what you like, but Google might cut you off someday.
A painful option is to have Duplicati move the files into the new account using a script as suggested in
Uploaded copy to google cloud breaks permissions
You could maybe keep using the Google Drive app after clearing the duplicates, however performance
could be different. Your description sounds like you’re streaming not mirroring (which adds other risks).