In the decades that I've been documenting my family tree, I have discovered several people that are what I call "double cousins". What do I mean by a "double cousin"? Well, to me a double cousin is someone who has more than one set of my ancestors contained in their tree which often makes their relationship to me closer than that of one of their parents.
Here's an example of what I mean: Her Majesty, Queen Elizabeth II, is my 19th cousin, once removed on my paternal side, through Her Majesty's maternal side. With this relationship, Queen Elizabeth's children are each my 20th paternal cousins. However, some of my royal 20th cousin's children are not my 20th cousins once removed (although some are); some of their children are actually my 18th cousins.
How is this possible? In the case of Prince Charles, who is my 20th paternal cousin, his first wife, Princess Diana Spencer, was my paternal 17th cousin once removed. This means that my relationship to the late Princess Diana was actually closer than that of Prince Charles. This then makes their sons, Princes William and Harry, my 18th cousins.
Both lineages come from different paternal ancestors of yours truly, but the common ancestor that Princess Diana and I share is a little closer than the common ancestor that Queen Elizabeth and I share. This closer relationship means that Prince William and Prince Harry share a closer familial relationship with me than the relationship I have to their father, Prince Charles. Confused yet? Don't worry, it can be confusing!
These different relationships doesn't mean that I am not also Prince William and Prince Harry's 20th cousin once removed - I am through their father, but through their mother, I am more closely related to them.
I ran across another famous distant cousin earlier this month - Julie Nixon, daughter of President Richard Milhous Nixon. President Nixon was my 30th paternal cousin, making his daughters my 30th cousins once removed. However, Julie married another distant cousin of mine - Dwight David Eisenhower, grandson of President Eisenhower and my 9th cousin twice removed, First Lady Marie Geneva "Mamie" Doud. Since my familial relationship to Mamie Doud was closer than to President Nixon, Julie Nixon's children are my 11th paternal cousins once removed. Their father, Dwight David Eisenhower, is my 11th paternal cousin.
Although at first confusing, putting together your relationships to various distant cousins can be a lot of fun. Luckily for me there are online resources available that automatically calculate the family relationships so that you don't go crazy trying to calculate everything yourself. Truth be told, if I had to manually calculate all these relationships I'd most likely take up drinking as a hobby!
Having more than one family connection to your distant cousins is not uncommon, nor does it mean that those sets of cousins were intermarried; it just means that through the passage of time, different sets of your common ancestors have met, married (usually) and had children. When you consider the number of people that were alive several hundred years ago, this makes sense as mathematically there simply were not enough people on the planet to avoid intermixing of families.
My relationships to both of these famous families doesn't mean that I'll be receiving an invitation to Balmoral or the White House any time soon, it's just another fun fact of what you can discover as you work your way through your own family history.