"Hello again" I don't think there is a set expression for it. If you meet someone again unexpectedly after saying hello once (I assume this is what you have in mind), you might say - 또 만났네(요) / 다시 보네(요) - 또 뵙네요 (more polite) They all mean simply "Hey I see you again". We never say 또[다시] 안녕하세요 - that sounds very strange.

When you are writing a formal letter: 다시 인사드립니다 (greetings again) When you are talking on the street upon meeting the other person again: 또 뵙네요 (here we meet again) 오랜만입니다 (long time no see (joking)) When you are message chatting: 리하이 (borrowed from English "re-hi") 리하이요 리하이여 리하이염 리하염 Well, that's called evolution!