Thank you two times

So, you're entering some sort of building, and there's two sets of doors. You walk into one, there's a small entry way, and then there's another set a few feet away. Say you're walking behind somebody and they open the first door for you. You say thank you, naturally. Then, they open the next door. Do you say thank you again? Do you have to repeat your gratitude or is it just implied that you are grateful by your first thanking? Is it considered bad manners to not say thank you again? I don't think a second full thank you is necessary. Some sort of secondary recognition is probably required. Maybe a head nod or something like that. The person opening the doors for you should understand that the first thank you carries over to the entire encounter. Saying thanks once establishes you as a polite person, and the door opener should be happy with that. I mean, they had to open the door for themselves, anyway. They don't have a lot of room to be snobby about it.