West Railway Station (Budapest) https://hu.m.wikipedia.org/wiki/Nyugati_p%C3%A1lyaudvar
The translation to English should be either 'I'm here again' or 'here i am again'. In this case you cannot say I'm after the here. This is a weird one because i cant think of many other examples. You couldnt say 'There I'm again'. It seems most the exceptions i think of end with 'again'!
"Újra" means "once again" while "ismét" means... Well, exactly the same. Nevertheless there are idiomatic usage differences that you have to learn or pick up while use the language. "Újra itt vagyok" and "Ismét itt vagyok" have a very tiny difference between them. Let me make up a situation: a frequent customer enters to your shop. You were expected him because you made an appointment, but as he arrived he realized that he forgot his purse in the car. When he arrives back, he will most likely say "Újra itt vagyok."
When he returns to you because the product you gave him doesn't fit to his previous holder, he will most likely say "Ismét itt vagyok."
These are very minute differences, and there is no problem if you mix them up, it may be even a personal style in most cases. It is very similar as a non-English speaker will learn the difference between quick, fast, rapid and their relatives :D (My English teacher ended up explaining this with "just get the hang of it".)