Steam Input
Steam Input is Steam’s controller layer. It reads PlayStation, Switch and generic controllers and presents them to games as a standard controller or as keyboard and mouse input.
It only applies to games launched through Steam. When a game also reads the controller directly every press can arrive twice.
In practice
Steam Input makes PlayStation, Switch and generic controllers work in games that only support Xbox controllers. It can also remap buttons or turn a controller into a mouse and keyboard. The common side effects are double inputs when a game also reads the controller directly and a stick that moves the mouse on the desktop.
How to check
Test the controller in the controller test with Steam closed to confirm the hardware works. Then turn on support for your controller type in Steam’s controller settings. If presses register twice in a game, turn Steam Input off for that game or turn off the game’s own controller support.