Another unenlightened misuser of words here.
I think maybe you have to live in an area where lots of 'dual heritage' people live to be fully aware of correct and PC terminology - and the eastern coastline of the UK has a very low population of ethnic minorities. (is that allowed?) Kent, Lincs, eastern yorks, Suffolk and Winchester are not areas where you tend to come across much more than the odd Chinese or indian takeaway. I have had quite a few friends of 'dual heritage', but as I never used any word at all to describe them the problem never arose, so was completely unaware that I was using the wrong terminology, something that those who live in more built-up areas seem to find hard to believe.
Maybe if I had liked gritty soaps more I would have been less ignorant.
I bet there will be another new word out next year and I shall be in trouble all over again. Apparently 'Native American' is also out of fashion. I had only just stopped using 'indians'!