What's wrong with that?
Infil: Yo, duel me.
Dude: No, you're too dishounorable.
Ex-infil: Hey look, I'm not an infil anymore!
Dude: Ok, let's duel.
*duels*
*One week (Yeah, class change only has a waiting period of 7 days last I checked) later*
Ex-infil: Kay time to put back on my infil cloak.
*turns into infil again*
You know, this actually makes sense in context. An infiltrator is someone who would reasonably know how to disguise himself, so he could at any time say he's a trader or something, duel someone, and slip back into the shadows. In fact, I'd say it's even a good tactic to use in order to avoid suspicion. After all, ICly people say that infils are too dishonorable to duel. This isn't unjustified behavior.
Of course, some infiltrators are notorious, which also matters because then the other character can just be like "No way, you're Snake Venomous, the evil assassin of the Empire" and simply refuse to duel you.