Keep track of which keys are down, and ignore keycode 39 until a keyup even clears it:
var down = {};
$(document).keydown(function(event){
var keycode = (event.keyCode ? event.keyCode : event.which);
if(keycode == '39'){
if (down['39'] == null) { // first press
$("#box").animate({"left": "+=30px"}, "fast");
down['39'] = true; // record that the key's down
}
}
});
$(document).keyup(function(event) {
var keycode = (event.keyCode ? event.keyCode : event.which);
down[keycode] = null;
});