It can be omitted, so you can write both 'en' and 'én'. But when you're using the accent, you're distinguishing between 'one' and 'a/an' more easily.
It's just like in Dutch, when you put an accent on the letter 'e'; when the Dutch word 'een' becomes 'één' like the Norwegian language 'en' to 'én'.
Either way, een means both one in Dutch. So more specifically; they are interchangeable.
We mostly use two diacritic signs (acute accent and circumflex), mostly in loanwords, person-names and geographical names. But you don't need to include them.