there are packages in the repos already which can accomplish this general task (duplicity, borg, freefilesync, probably several others) - is there something special about Duplicati?

IIUC it performs the same function but in a different more efficient way. In comparison reviews I've found it appears to be rated higher. All subjective of course.

ok can you find a working PKGBUILD for it ? - and are you willing to keep an eye on it, report bugs when it breaks, try to upgrade the PKGBUILD when necessary, or otherwise be the maintainer?