If you pass the same people asking for money everyday. are you obligated to give them Tzedakah each time you pass? how does it work?

You mean you pass them once a day, every day, or you pass them multiple times each day, let's so going in and out of the store?

If it's on Purim, I believe you are supposed to which would imply that you are not required to otherwise. (I think that's how it works.)

Personally, I don't. I like giving on the way out, that way I know I gave, but feel better about it (also, I usually ask for change on the way in, since my kinderlach and I empty out the change every erev Shabbos for the pushkah).

However, I do not know what the obligation is.

Well lets start with passing them once every day.