Hi, I would appreciate if someone could explain how entry/exit triggers work on worklows - in NS user guide it clearly states that those triggers invoke an action only on the first time that a workflow enters a state but in our NS entry/exit are triggered again if the if the state has been exited and then entered again (imagine a loop structure of states). What am I missing?

Not sure I understand the question but I'll give it a go. When you load a record - actions in the current state of that workflow that are not entry/exit often occur every time you load/do something to the record. (think before record load, before record submit, after record submit) Entry exit only occur on a transition from one state to another. Either entry - as it transitions into the state, or exit as it leaves from that state into another. If you have a loop in your workflow that causes a record to go through multiple states- any entry/exit actions will reoccur based on the previous paragraph.

What you said is the beahviour I see in Netsuite but in the user guide it says: If the records enters the state a second time, the send email action does not execute again. Am I missing something?

I've got several workflows that rely on intentionally looping things. Trust me, it works the way I describe.