Duplicati doesn’t have its own SMB client – on the source side the OS sets things up, then it’s just files. Going with what @drwtsn32 suggests will have those files be accessible faster, e.g. for change scans.
If you can’t do that, then perhaps some additional security could be had by having Duplicati use a user account that’s not used for anything else – I believe drive mappings, e.g. by net use are kept per-user.
UNC source with credentials? talks about this. Windows seems to not directly do UNC-with-credentials. Possibly someday a Duplicati enhancement would let it do source drive login. I don’t think it can now…