@GreenEggsAndShame I am really sorry this is happening to you, it's boring and tiring and hurtful.
I have mixed SA heritage and i get it. There is a casual racism inherent in it, however much all the white people on here seek to diminish it and say they have had the same experience. It is different because there is a power dynamic involved and you are racially othered and minoritised in that space.
I have had the same - i work closely with a colleague who has the same heritage as me - she couldn't actually look more different- but we are the brown ladies - our names are regularly swapped in emails and in person and people, even close colleagues seem to think we are interchangeable - have the same specialisms, opinions, interests etc. It is such a joy when someone takes the care to ask how to say your name and to remember, and spell it right.
I see it play out all time though - the assumption that there is a homogeneity in all non-white others.
For those who don't agree or don't see this behaviour as having a racist element, I would really really encourage you to read Layla Saad's "Me and White Supremacy" laylafsaad.com/
Your experience is not the experience of the OP and this is one of hundreds of daily behaviours that happen over a lifetime of being a person of colour. Sometimes its too much, so when you say - oh its normal, get over it, don't be sensitive- do try and consider the constant drip drip drip of behaviour like this. And yes not every person of colour deals with it or feels it in the same way, or even feels the same every day, but sometimes they do and it's entirely valid.