The game allows users to shoot using both Z & Insert (both Insert keys work). You can use this to double your shots. Very useful on the final boss since it takes so many hits.

This a quick reminder that one-to-one keybinding is allowed.

That means you are allowed to assign the default controls to different keys or a controller. However, every default button must be assigned to a unique button. For example, you cannot assign both Z and Insert to a single X button on your controller or keyboard.

On a controller, you can use Joy2Key to assign each action separately.

You can also use this to reassign the keys to other keys that you find more comfortable.