Letter Quest: Grimm's Journey Remastered

Like the example shown in the achievement description, a double-letter word is a word with the same letter back-to-back. Even though it shows "bubble" as an example, double-letter words with consonants will likely not be as common as double-letter words with vowels. So keep in mind if you've got a double 'E' or a double 'O' that those can be used to make double-letter words as well. As long as you keep this achievement's requirements in mind while playing, it should easily come naturally while playing and you shouldn't have to force it.