/hud DutyList
or
/hud dlist

You can also add "on" or "off" at the end
/hud dlist off
/hud dlist on
but if you don't specify the "on" or "off", then it's just a toggle.

If you want quick access to it without typing (particularly useful if you play by controller), then put that
/hud dlist
into a macro and assign the macro to an action slot on one of your hotbars.

You can find other useful commands at the Lodestone's Glossary of Text Commands page. The "/hud" one, for instance, falls under the "Other" category in the category listings on the left side of that page. There are quite a few other things it can show or hide as well as just the duty list.