I feel your pain. I actually ported Duplicity to Windows about 3 years ago for a client, but the code was never merged (or commented on).[1]
In the end, that client decided that it was easier to use a commercial package, given their typical use case. Tried Carbonite (didn't work, support unhelpful) but settled on JungleDisk. YMMV.
1. https://code.launchpad.net/~kevinoid/duplicity/windows-port