Function in Clojure that takes a list of n letters and returns a list

I need your help to write a function that takes a list of letters like ['a', 'b', 'c'] and returns another list of letters like ['a', 'b', 'b', 'c', 'c', 'c'], this is the first element is repeated only once the second element is repeated two times, the third element is repeated three times, etc. Thank you in advance!.