If a mission has already been assigned before, let it be labelled “Assigned”. At least you wouldn’t have to spend 40 minutes clearing a max difficulty + max length mission on SSH to assign it, only to find out that you’ve already assigned it before. People will eventually forget that they have already assigned that specific mission.
You have no idea how many times this has happened to me, completing a mission with intents of assigning it to my squadron, only to be told: “This mission has already been assigned.”, after spending 20 minutes on it.