Not sure I worded that the best way 
I don't know how, but somehow each year MIL has bought DD something exactly the same as I have gotten her, it happens both Christmas and at birthdays. She then gets annoyed and refuses to return it and gets a bit rude to me for getting the same thing, as if it was planned
I don't know how to politely say 'oh by the way, we've bought DD XYZ so can you buy her something else?' But when it's her children (DH has a big age gap between him and siblings, they're more like nieces/nephews) I always ask 'is this okay, do they have it already' etc. I thought that was pretty standard, to make sure that you're not buying duplicates? In future, should I just tell her what we're getting DD (for Christmas, birthday etc) to avoid her getting aggravated?