Just in case you haven't figured it out yet, change the Input.is_action_pressed("ui_up")
to Input.is_action_just_pressed("ui_up")
The reason it didn't work is because Input.is_action_pressed("ui_up")
checks if the button is being held down, thus when you press it, the first and second jump gets executed at the ame time, making it look like a single jump.