Creating an Isogram Checker

I am currently working through the swift coding challenges posted on this website, and I am currently on the isogram checker. So far I have turned the word into an array, but I have got stuck trying to find a method to iterate through the array to check if letters are repeated. I have tried a for in loop but can’t figure out a way to reference every other letter in the array to check against. I also have considered a switch loop but again can’t figure out how to lay it out. Any help would be greatly appreciated!! Many Thanks!