Assuming common agents, is there any reason to have more than 1 of each? i know iconic only one can quest, is it the same for common agents? ie best to convert all common duplicates?
As a general rule I would, maybe just keep 2-3 of each Job Type on top of that, they become progressively less useful as you level up, as all job types begin requiring higher "Skill" levels from the agents to do the higher level quests. I'm at the point where the only use for my commons is generally recruit quests which penalize you for using higher tier agents
Oh, with regards to your question specifically about the iconic thing, you *Can* send multiple of the same common out on separate quests. But converting the extras should give you better and more versatile options
Commons are very good for filling secondary slots, since a good number of quests start specifying "common X" as the preferred agent, but you can treat overseer like pokemon and sluff off all duplicates.
"Retiring" produces an item called an Echo, which give you a chunk of xp towards leveling your overseer quest types. Not enough stats to really comment on their value, I retired an uncommon to see what I'd get going from 4 to 5 Research and it gave me about 6% Research XP. I imagine it'd be more substantial for level 1 to 2
Retiring also lets you convert that Uncommon+ agent to 2 lower agents (it's the last choice in the Echo reward). Particularly if you are not wanting to pay anything for Overseer, it can be another way to get a type of job you do not have. You lose one in the process (i.e. common -> uncommon takes 3, uncommon -> common makes 2) but if you have a recruit mission you simply cannot do, it can be an option.
Retiring higher rarity agents is very good overseer XP, and can be the best way to raise it bars at higher levels.