How to check if user input is typed before?

I have created a hangman-game.

I want to ask how I can check if the user type the same letter. Everything is working fine but without checking if the number is allready typed.

if (word.contains(input)) {
        i = i - 1;
        char[] charOfGameBoard = gameBoard.toCharArray();
        for (int x = 0; x < word.length(); x++) {
            if (word.charAt(x) != input.charAt(0)) {
                continue;
            }

            int j = x * 2;
            charOfGameBoard[j] = input.charAt(0);
        }

        gameBoard = String.valueOf(charOfGameBoard);

    }

I cant post to much sry.

word = "house"

charOfGameBoard= _ _ _ _ _

userinput= "h" //Correct

charOfGameBoard =h _ _ _ _

userinput= "h" // incorrect at this point

charOfGameBoard =h _ _ _ _