In this case you don't need an else. If you click the button the first time, the condition (left side) will be true, so the actions (right side) will be executed. The second time you click the button, the conditions will be false, so the actions will not be executed.
A different story would be for example if you want to play a sound that is either "success" or "error". In that case you want a specific set of actions to play depending on the conditions. So you'd do
if not cow in array -> add cow to array, play success sound
else -> play error sound