The world is not one of exactitudes of ethnicity, but rather one of fuzziness and blurring between groups of people, especially those living on the borders of countries.
Borders are illusions that have no geographical reality, but are constructed out of fear and perceived cultural biases/similarities. However, the truth is that humans have always been a nomadic species, transmigrating across the globe and intermingling, long before there were laws, visas and passports. This is most certain to continue, whilst we remain guests on our planet. Indeed, there is every likelihood that humans may one day migrate to Mars and beyond.
There are two ways to respond to this reality. The first is to take the response of Nationalists, who fear that there will be some sort of cultural/ethnic dilution of their group, if there is an influx of people with customs and ethnicity different to their own. And, most certainly, this is a two-way state of affairs, that can manifest on both sides, bringing with it racism, violence, segregation and hate.
The second and more preferable option, and the one that seems to wholly embrace the inevitable, is to just appreciate that everything is in a natural state of flux, including culture. Accept that nothing remains the same forever, including ethnic identity. Embrace diversity and look for common humanity in all.