Welcome to the forum @forcam48
Are you running Duplicati as a Windows service? That doesn’t see the drives the users do, and fixing that is problematic according to Microsoft. UNC source with credentials? suggest Active Directory can help, but do you have it available, or are you just having people type in credentials, maybe with Credential Manager help?
Services and Redirected Drives is Microsoft information. If you’re not a service, then your life probably gets easier, however you still need to find some way to get the share set up, perhaps using a –run-script-before.