how to add duo principals in mfa script

[Jim Fox]

> Duo has a "remember me" option that will do what I think you're asking for; when a user first authenticates they get prompted for Duo, subsequent authentications "remember" that they've already done so and so are not prompted again (within that specific browser session)...
> https://duo.com/docs/remembered-devices
>

There are a couple of reasons that won't work for us:  We might require, 
at least for some SPs, that force reauth means no free ride for the 2nd 
factor; and we might need more detailed information regarding who uses the 
feature and how often.   Stuff like that.

Jim