My name is Janet Ann Collins, but many of my friends call me Jan.
Nobody called me that when I was little because, due to World War II, everyone knew Jan was a boy's name in Holland.
When I was a kid I only knew one other person named Janet until the movie star, Janet Leigh, became famous. Then hundreds of girl babies were named Janet. I'm in several writers' groups and all of them have at least one other member named Jan or Janet.
It's amazing how many other people have similar or related names. John, Jane. Juan, Shawn, Sean, Shanna, Juanita, Hans, Giovanni, Zane, Ivan, Jeanette, Jean, Jonathan, Janice, and many others are all related. (In case you can't tell, I've always been interested in Linguistics.)
I understand the original version of those names was Hebrew, and the meaning was something like "God gives grace," ""God is gracious," or "Gracious gift of God."
With a meaning like that it's no wonder so many people chose similar names for their babies. Of course, as the name spread through various cultures and languages, lots of parents were unaware of the original meaning and simply named their children after other people.
It's no wonder there are so many of us in the world!