Remove duplicate key from array push using textContent

Question

I have a an array that writes letters from a-z and it writes duplicate values to my html, I want to prevent it from writing duplicate values how do I do this
Document.onkeyup = function (event) {

If (event.keyCode >= 65 && event.keyCode <= 90 && gameStarted) {
LettersGuessed.push(event.key.toLowerCase());
LettersGuessedHolder.textContent=lettersGuessed.join(‘, );

}
}

So letters guessed goes into that array .join but I want to omit duplicate entries any suggestions. Please

Answer 1

you can use the native filter method of an Array.

Answer 2

I bit more detail would be lovely like a actual solution being that am new to JavaScript