Duplicate Minecraft Java Edition version

That's intended. The one at the top is the latest development version. When you select that one, it will always update to the new one when a new snapshot or pre-release comes out and the on below is just a normal one to allow you to play the snapshot when it isn't the newest version anymore. When you select that one it will always stay at 21w17a even if there is a newer versions. That's just how versions on the launcher work.