Are they really duplicates? ie. - does the mojo.robofont function actually just call the equivalent in robofab?

And if so, what is the guideline for when to prefer one module over the other?

the guideline would be:

• use robofab.world if you want to have your script interchangeable with NoneLab, FL and others.
• use no import: all mojo.roboFont functionality are added to the script locals() (works only when you run the script in the scripting window)
• use mojo.roboFont if you use your script as a module only in RoboFont

http://doc.robofont.com/api/robofab-extras/

1. These are exclusive to the Robofont environment.

2. They are not part of any Robofont-native module, for instance mojo.robofont.

3. They are only available by importing robofab.

4. But though they are robofab-based, they will not work in environments other than Robofont.

It doesnt matter if you imported it from mojo.roboFont or robofab.world (inside RoboFont) all robofab object will have these additions.