For part-time duties? Pretty much the same way you always do with other traits, I'd imagine. Like when part of the PCs has wings and part doesn't. Or when one PC has an Enemy. Or like when not all PCs can swim. Or when some PCs have additional objectives in a mission.
For full-time duties? I don't think a full-time duty is appropriate unless the campaign is set in a garrison or the like. Obviously Disadvantage pick appropriateness varies by campaign.