OK – (for others in a similar bind) I solved the problem by first installing EPEL (http://mirror.cs.princeton.edu/pub/mirrors/fedora-epel/7/x86_64/e/) and then I was able to explicitly run
yum install libappindicator
Once I did that, I was then able to install Duplicati by first downloading it and then running
yum --nogpgcheck localinstall duplicati-2.0.2.10-2.0.2.10_canary_20171011.noarch.rpm