Of course I thank some-one in front of me who holds the door open for me to pass, but if the same person holds open multiple doors, should I thank them each time, or just the first and last time - and how can I know which will be the last?
I am regularly filled with sorrow when I read such letters as this, demonstrating how low the manners of our population have sunk; however you did right to ask me!
You should not only thank the person at each door, but before you pass through the door you should both put your hands together and bow as a mark of respect.
If you both bow low enough the door will not be able to slam in anyones face.
If one does not bow low enough, not only will the door slam in the face, it will also be a sign of bad grace and so in such circumstance it would not be appropriate to apologise.