The Two Door Dilemma

Etiquette question: You walk up to a building slightly behind someone. There are two consecutive sets of doors. The person after walking through the first door holds it open for you. Being polite, you say, "thank you." The person then walks through and holds open the second door, are you obligated to thank him or her again? Is it one kind act or two? Is holding the second door a continuation of the thoughtfulness of the first act and thereby covered by the original "thank you" you uttered two seconds earlier or do you need to say it again? Does the fact that the two are the same kind of act mean anything? Would a smile instead of a verbalization count?