No double sneak is a minecraft fabric mod that fixes MC-159163, a bug where if you quickly sneak and unsneak on a server it can "bounce" your camera.
This mod is made to be as barebones and simple as possible to make it really easy to update. In fact, this would not exist if Sneak Tweak (another mod that adds customization along with the same fix) was up to date and didn't depend on a library that wasn't up to date itself.