Hi Jessica! Nice to see your message. I'm not exactly sure, though, what you meant by King Niall being repeated. If you meant that you are descended from him on several different lines, that is not unusual (although it should be more than 6 generations ago, i think). It is called endogamy, and was quite normal in the past when people stayed within a smaller geographic area.
Here's a little article about it. https://isogg.org/wiki/Endogamy
Or maybe that wasn't what you meant?
Best regards
Shirlea