In any system with a lot of skills, you need to allow for some overlap.
Whilst Duping, Acting and Seduction may all be used to sell a lie, I don't think they overlap as much as they each match different ways to do so.
In the context of selling a lie to guards so that they let you go, using Duping is fast talking them, including with lies and half truths, so quickly and confusedly they don't have time to think about it, pretty much ending with a quick "...and so, I have to go, good bye" and leaving. It's the classical and comical situation where the hero quickly leaves, just to hear the guards call him, "Hey you!", and thinks, "Damn! They realised already!" Advantage: it doesn't take time so it's the best way if you're in a hurry. Drawback: as soon as the guards have time to think about it, they
will realise you lied and duped them.
OTOH, Seduction would be to manipulate the feelings and emotions of the guards, gaining their sympathy, or pity, or anything of the like, so that they'd let you, including actually seducing them. It's the case where they'd tell you, "Okay, I let it pass this time." Advantage: if it works, the guards willingly let you get away, so there's no chance (save external circumstances, such as a higher officer passing by and wondering) they'd go after you. Drawback: most of the time, the guards realise you're lying to them whilst trying to gain their pity / sympathy / affection / whatever so it all eventually depends on their will to let you go, which is less likely to happen the bigger the lie is.
At last, Acting would be indeed
acting as if you were honestly believing your own words. Advantage: even if you fail, you may still act as the victim who was fed the lie, thus lower your sentence. Drawback: of the three, it's probably the one that takes the most time, as you have to, calmly and politely, discuss with the guards, and they usually have time to think about it and realise it's a lie.
In other words, in my opinion, someone skilled in the three skills should:
* If it's a small lie, try Seduction, because even if the guards realise he's lying, they'd let him go because he's cute / pitiable / likeable / whatever compared to how minor the lie is,
* If it's a big lie and he thinks he's able to outrun the guards, try Duping,
* If it's a big lie and he doesn't think he's able to outrun the guards, try Acting so that, if the guards don't know or don't realise what he says is wrong / absurd / whatever, he's able to leave but if they do, he'd get a lower sentence because, honest! He didn't know! It's what he was told!