I found a possibility if you have only one instance of Duplicati: NetLimiter 4 for Windows.
Unfortunately not freeware but it fits for me! The Pro version costs about 30$. Maybe the Lite can do also the limitation then 20$.
You have many possibilities. You can limit the internet speed of the process (duplicati.gui.tryicon.exe). You can set this limits “by hand” manual or with rules => eg. enable the limit 23:00 and disable it 06:00 or you can limit it by target (eg. 126.96.36.199 for the Cloud of Deutsche Telecom) or you can set priorities (I have not tried this yet) or you can set limits after reaching up/down quotas (also not tried this).
Here an example for a limit set just for duplicati internet traffic:
And in the process view you see the upload limit 80kb/sec
And here a limit for a specific cloud server is set, no matter if the traffic is from Duplicati or a different process:
If you have more then one PC with Duplicati a possible solution would be to redirect the traffic of all Duplicati-PCs thru a proxy and then use a proxy which can throttle different users. Eg. CCProxy (costs about 4$/year).