Hi Ralph,
That capability does not exist yet. You might want to go to
duplicity issues and make a formal request for the feature.
I've seen something like this requested before, but the difficulty to implement is moderately high. If you feel up to it, feel free to contact us and we will help where we can.
For now, a full backup every month is the way to go. A long string of incrementals is
not recommended.
I'd keep the last two full backups in separate places (local and remote) for safety. I'd also use verification frequently.
...Thanks,
...Ken