I have an idea trying to worm its way through my head, and it's presenting me with what appears to be a relevant question regarding the legal, moral, and ethical positions on an incident.

Specifically, in this case, a disguised changeling ('Mimic') is trying to get Filthy Rich to sign a contract. She began the conversation, though, by issuing two commands to Filthy via changeling mind control. The first command is that he would find nothing in the conversation to be out of the ordinary; the second was that, once she walks out of his office, he will forget that the conversation ever occurred, at least until such time as Mimic (or another changeling playing the same pony) reminds him.

At the time the contract is signed, Filthy Rich is technically under mind control. Normally, I would expect that to count as duress, thereby invalidating the contract. However, the mind control only barely affects his reactions; he's finding normal some things which might, under other circumstances, strike him as being slightly strange, but everything else is supposed to be as usual. Under these circumstances, does this still count as duress?

6049063
Yes. For a number of reasons.
(1) If he would find something out of the ordinary about it-- like doing business with a changeling-- then he is under influence. If he wouldn't make the contract knowingly, or if the terms would be different, etc. the changeling has engineered an advantage. It doesn't matter how 'slight' it is. The changeling is literally creating a compulsion necessary for the contract to exist.

(2) The reasons why he might find this out of the ordinary are many and varied. I don't know your version of the world, but it could be anything from treasonous to questionable to even enter into a business relationship with a changeling. It could be his personal prejudice. It could be a consideration for his business partners and market. It could be a suspicion based upon the fact that he's doing business with people whose identities he will have a hard time establishing and who can apparently control his mind. By removing that, they are literally impairing his judgment. He is literally under compulsion. Most legal and ethical systems specifically try not to get into the weeds on 'how much' compulsion' at the point that you're allowing that the compulsion exist at all.

(3) The changeling is making review of the contract impossible unless she or someone similar permits it. So now they're not just forcing him into a situation that he might not enter into under ordinary circumstances, they're making it impossible for him to self-determine if he's been cheated or compelled without their permission.

(4) Specifics of functional magic aside, what you're describing is altering his psychology. If there's any conflict between that and his personality and history, it will create conflict, possibly even psychological stress. This is a bit weaker, because it relies on the magic not magically filtering out everything except the specific conflict, but you're hanging a gigantic conceit on it being that effective and having no traumatic or stress induction despite the situation being grave enough to require the mind control in the first place.

Of course, fiction being what it is, protagonist-centered morality isn't always a bad thing. It just means that repeatedly going to the well will stretch suspension of disbelief, and make external observers wonder if they're really looking at a hero, or even someone that they should be sympathetic to at all. So you CAN have Filthy be ok with this-- especially if he decides it is in his interests to continue-- and the contract can function as an agreement. And there's nothing that says that your version of Equestria doesn't permit certain amounts of compulsion, or maybe you can determine magically how much compulsion counts as "really" putting someone under unacceptable constraints.

But speaking from at least my point of view, if you told me, as a reader, that the court system of your Equestria would determine that a contract that wouldn't exist without the compulsion is valid, I would start to consider your Equestria a dystopia, or to have dystopic leanings. I would say that even if you can determine "what makes compulsion too much," who are you having examine that? Who determined and by what basis did they determine that it was okay to change someone else's mindset because you didn't change it by over that certain amount? And how is it possibly fair if he can't even remember it for analysis, review, or discussion with others? Each time you go to "It's magic," on those, you're leaning on the wall a bit more heavily. It will break eventually.

6049063
I think it comes a bit down to how the magic in question specyficly works. The first command is the key. Because it might even not affect Filthy's reasoning. From how I read the command is that the changeling applied it to keep her disguise 100% fool-proof. Because if she didn't she couldn't sign the contract as she must sign as herself and she can't do that if Filthy does recognize her as a changeling. She would start the conversation as a crafted identity and gradually switch to her normal one so she can sign the contract to make it legal when at the same time unabling Filthy take note of her true identity.
Then things get tricky. Becuase if the contract is binding depends on what it is regarding to. The contract must work off the assumption that Filthy signs it with a newly met person, all the info provided to him was true (besides race which he knew without reacting to it) and race of the other signitary didn't play a role in the contract. Besides otherwize it's eather exploiting Filthy or him discriminating based on race/species.
THAT ASSUMES THE CONTRACT ITSELF ISN'T SOME SORT OF SCAM, IN WHICH CASE FILTHY WOULDN'T SIGN IT.

As for the 2nd command - it's an outright crime. It's hiding an important information of a legal occurance who Filthy was part of. Repercutions for that are sure to occur but it might not affect the signed contract as Filthy was still able to refuse signing it, still assuminng that the spell was there to not have Mr Rich call guards on assumption that she is an enemy combatant. For it she might spend some time in prison ( a year maybe, with the option of early relise), pay a big penality fine (preferably scaling with her incomes and possesions) or have her part of the contract be modyfied to be more beneficial to Filthy.

So in short I think the duress doesn't occur but signing such a contract can't be done without commiting a crime. If it ends up in court and Filthy was proven to had part of his memory wiped then the changeling would face charges for that but the contrct would stay in power assuming the command works like it sounds. As in would block Filthy's eventual reaction to her beeing a changeling which would be rasist, so it actually doesn't matter as she was securing herself against a crime commited against her.
A form of self-defence she can be excused of as her kind is always weaker than any other equestrian citzen and has to rely on bending the law to stay afloat. And if anyone were to argue that she cannot that would be oppression. Changelings are weaker than any other race with flight, magic and phisical cappabilites, need to consume love as well as lack CUTIE MARKS THAT GUARANTEE SUCCESS IN AT LEAST ONE THING. Their only advantages are beeing able to use flight and magic at the same time, shapeshifting and psychic abilities. And using those advantages is ingrained in their nature so forbidding them using them is open discrimination.

Furthermore, Filthy must possess a copy of the Contract of his own. Even hidden somewhere between his documentarion so that he wouldn't find it for a long time but I imagine documents in Equestria can actually be enchanted to easily find them (making them float, light up or habe a compass lesd to them) to prove of it's egzistance if needed. He still owns it and would make it valid.

I'm sorry for my rather simplistic wording but english isn't my first language.

Well, in a court of law, anyway, if any mind-control-foul-play was even suspected, there'd be no way to prove the severity and type, the court would have to assume the worst, and invalidate the contract.

But like, if he forgets about the contract anyway, isn't that a good enough lamp shade?