Absolutely not!
I've never heard of 'Matching Mothers' at a wedding! Here in the US at least, it's even considered a 'faux pas' when the Mothers show up wearing the same color or Heaven forbid, the same dress!
No need for a confrontation, but do let your DIL know (the sooner the better) and in no uncertain terms, that you already purchased your dress and you won't be returning it. It's been fitted to your size and figure, it's very flattering, you love it and that's the dress you will be wearing!
Harry and Meghan ‘royal’ tour of Nigeria.