Duplicate letters in a string

Write a function findDuplicateLetters(text) that finds the duplicate letters in text: the letters that occur more than once.

• Return an array of letters in lower case.
• Case does not matter (case insensitive): A and a are the same letter, so 'Aa' gives ['a'].
• Only the Latin letters a-z count. Digits, spaces, other characters and letters outside a-z (like é) are ignored.
• Every duplicate letter is in the result once, even if it occurs three or more times.
• Order: by the first occurrence of the letter in text. 'Programming' gives ['r', 'g', 'm'], not ['r', 'm', 'g'].
• If there are no duplicate letters (also for an empty string), return an empty array.

A simple way is a counter for every letter in a Map, then a second pass over it.