Hi Donna,
Are you asking for help with the Duo configuration, or with the Fortinet steps?
You can use the same Authentication Proxy RADIUS configuration with [radius_server_auto]
described here with IPSec connections.
Then in the Fortigate you should be able to specify that Duo server as the RADIUS server for remote IPSec users.
I haven’t set up Fortigate IPSec myself but feel free to come back with more questions.