Yes, it’s a nickname. But it’s the name I wish to be addressed by. I don’t get all het up when someone messes it up after only meeting me a few times, or when it’s a vendor or if it’s someone who knew me before I started the joke. People in the US have difficulties wrapping their heads around the doubled first names because it doesn’t fit our government forms system.
But if you’ve been working with me for almost four years, you damn well better call me Mary Sue.