So, the issue really isn't the Christmas present thing, as much as it is the lack of respect for the boundaries you & DH try to set, which MIL does not adhere to, and the favoritism shown to one child over the other. That wasn't quite evident in your first post, but became more evident with subsequent posts.
So, what to do?
You need to determine if this is a hill you want to die on, if DH will be supportive of you in doing so and how far you want to go. Frankly, without DH's support, you're not going to get anywhere. So, discuss that part with him first.
If it's a hill you both decide you want to die on, then whatever gifts Grandma gives that are repeats of yours, that you requested she NOT get your children, you ask her for the receipt so you can return them. This will cause the ensuing drama/trauma, with manipulation etc., probably a family fight involving DH's other sibs, your FIL (if he is alive) and perhaps extended aunts & uncles. It might be worth it to you it might not be. Only you & DH can decide that. And with each phone call regarding said trauma, you will need to learn and continuously repeat the same "mantra", without veering from it, without arguing, without losing your cool, without feeling any need to justify anything any further beyond your "mantra"...something to the effect of "We told MIL that we were purchasing XY&Z for Billy & AB&C for Susie, and she purchased duplicate gifts anyway. The kids certainly don't need 2 of them, so we are returning hers and will purchase something else". The ONLY successful way to deal with manipulation, regardless of where it comes from, is by repeating the same response until the manipulators learn that no matter what they say or do, you aren't changing your mind. This advice comes from years of dealing with substance abusers as a nurse and a super-crazy & super-manipulative SIL.
Of course, if you want to make your whole life a lot easier, then do the "Grandma List" thing for Christmas, and tell the kids that whenever Grandma asks them what they want, they should tell her "what we put on the list we gave you".
With regard to the favoritism (which to me would be the bigger issue but that's just me)...that's very simple...if Grandma doesn't give something to both kids, then she doesn't give something to one, with the exception being their birthday, which is an individual celebration, unless, of course, they are twins. And the gifts must be equivalent, because Grandma sounds like the type who would bring the favorite a Wii and the non-favorite a piece of gum and then say "Well, I brought them each something". So, the "surprise I love you" gifts need to be "equivalent". So if DS gets lounging pants, DD gets lounging pants or a cute top or a fancy hair doodad...you know what I mean...something that a little girl would consider equivalent to her brother's lounging pants. DS does not get lounging pants while DD gets a sticker. And if that did not happen, then I would, unfortunately, take the item that Grandma gave DS and return it to Grandma, while saying to DS "I'm sorry Billy but Grandma can't bring you something really nice without bringing Susie something really nice and as often as we tell Grandma this she doesn't seem to get it. I know you understand that it's not fair to Susie that you always get gifts when she doesn't. Maybe someday Grandma will remember that we love you both equally and treat you both equally". If you have raised your kids as well as it sounds like you have raised them, then Billy will understand, even if it might be a little hard for a 10 year old. But kids are pretty smart...you'd be amazed.
Again, the key is to remain calm and cool, as soon as you lose your cool, you have lost the battle. And it will be a battle, until the "new normal" that you are proposing becomes a habit.
Please don't think that changing these behaviors is going to be easy. It will not be. Once my late DMIL died, my crazy SIL took years to get under control (DMIL was her enabler, always letting her get away with stuff to "not cauise a family rift") to the point where one could have a reasonable conversation with her, and that was after me literally telling her that she added nothing to my life and if she didn't stop that I would write her off as if she never existed. And even then, her first repsonse was "I don't think you have the nerve to do that" to which I responded "I don't think you have the nerve to test me". I was right, she hasn't had the nerve to test me. I had also found her "Achille Heel" which was the fact that she did not want to lose contact with her family. But it took many years and many confrontations, discussions, attempts at manipulation etc.
It's quite draining. OP. I wish you well....
Whatever....like water off a ducks back, just let it roll off. She'll get over it and maybe eventually LEARN to back the heck off like you asked. If she doesn't. Too bad for her.
You got it sistah!
Now you're talkin'!
It's unfortunate that you think there could have been a motive to my comments other than an alternative perspective. 
