Earlier quoted context omitted.
In this particular case, I'm guessing -- based on the URLs -- that in order to have the Instagram username "sussexroyal", it's also necessary to be located at www.instagram.com/sussexroyal . This is a design mistake. The URL identifier does not need to be the public-facing identifier, and it shouldn't be. You can have the Duke of Sussex as sussexroyal at www.instagram.com/y6llflk9 and the guy in Sussex who roots for…
But instagram.com/sussexroyal is a much more memorable URL than instagram.com/y2q9g6uo, and being able to find someone from remembering their handle is much better than remembering the handle and then wondering which of the multiple people called sussexroyal you're after when the search results come through Needless to say, the most prominent and memorable identifier being non-unique has all sorts of uses for trolls…
Sure, but it has more uses for people who would like to have a reasonable name. This is just the "knives can be used to kill people" argument.