doubleLetters (x:y:xs)
  | x == y    = True
  | otherwise = doubleLetters (y:xs)
doubleLetters (x:y:xs)
  | x == y    = True
  | otherwise = doubleLetters (y:xs)
doubleLetters (x:y:xs)
  | x == y    = True
  | otherwise = doubleLetters (y:xs)