There might be a typo (“Note” as “Not”) in the report from @JonMikelV but I wonder if those Duplicati service processes are two Duplicati.WindowsService.exe plus a Duplicati.Server.exe? Which ones restart at activate? Viewing in Process Explorer is easier (plus one can see where executable is from), but Task Manager suffices. Process Monitor is a good way to see the exit code attempt (which goes to a spot that doesn’t process those).
Here, the Duplicati.Server.exe is just restarted in its original version by the child Duplicati.WindowsService.exe because the exit code it uses to request an update-restart only works between a paired child and parent, e.g.:
Duplicati.Server twice? (which is maybe not quite correct because it can also just pass the exit code upwards)
[SOLVED] Is it ok that I see 5 processes of Duplicati in Windows Task manager?
I don’t know if it was intentional for Windows service to have a single Duplicati.Server.exe as well, but see this, and another possibility might be for the child Duplicati.WindowsService.exe to return special exit to ITS parent, thereby starting the newer version of Duplicati.WindowsService.exe and (I hope) the new Duplicati.Server.exe.
The only way here for a Windows service Duplicati to activate the update is to restart the service, reboot, etc.
Starting a Windows.Server.exe from the Command Prompt causes the paired parent/child, and activates fine. Stopping with Control-C is not so good because it only kills the update-runner parent, leaving its server child.