Are you sure this isn’t Dropbox filling the C: drive with the files that Duplicati is uploading into Dropbox?
How to change desktop app preferences talks about how to set which remote folders you want to sync.
If for some reason you’d rather have everything sync to D: drive instead of the C: drive, these may help:
Moving your Dropbox folder to a new location
How to Change the Location of Your Dropbox Folder
Setting up with Dropbox sounds like you were asking Duplicati to back up to Dropbox, not to local folder. That’s your actual backup, and if you’re inadvertently getting a local copy back, Duplicati doesn’t know it.