Solution

<script>
 previousClick = undefined;
 previousClickTime = undefined;
 clickCounter = 0;
 function buttonClick(letters) {
    var input = document.getElementById("put");

    if(previousClick) {
    if(letters[clickCounter == 0 ? letters.length-1 : clickCounter - 1 ] == previousClick) {
        var now = (new Date()).getTime();
        if(now - previousClickTime < 500) {
        var val = input.value;
        input.value = val.slice(0, val.length - 1) + letters[clickCounter];
        previousClickTime = (new Date()).getTime();
        previousClick = letters[clickCounter];
        clickCounter = (clickCounter + 1) % letters.length;
        return 0;
        }
        else
        clickCounter = 0;
    }
    else
        clickCounter = 0;
    }
    previousClickTime = (new Date()).getTime();
    previousClick = letters[clickCounter];
    clickCounter += 1;
    input.value = input.value +  previousClick;
}
</script>

<input id="put" type="text"></input>

<button onClick="buttonClick('abc');">abc</button>
<button onClick="buttonClick('defg');">defg</button>