We're chasing a different MultiTap+Tap bug but this particular one behaves as designed. Each interaction goes through a cycle involving a started, a performed, and a canceled phase. When LMB is press, the MultiTap interaction sitting on top of the stack is started and thus a callback on the action is received. When LMB is then released and not pressed again, the MultiTap cancels (without performing) and thus another callback is received.

If the script is modified to read like the following, the MultiTap is never performed except if LMB is in fact double-clicked.

public void Fire(InputAction.CallbackContext context)
{
Debug.Log("Fire!");

if (context.performed && context.interaction is MultiTapInteraction)
Debug.Log( context.interaction );
}