Check if class already assigned before adding

Just call addClass(). jQuery will do the check for you. If you check on your own, you are doubling the work, since jQuery will still run the check for you.

A simple check in the console would have told you that calling addClass multiple times with the same class is safe.

Specifically you can find the check in the source

if ( !~setClass.indexOf( " " + classNames[ c ] + " " ) ) {
  setClass += classNames[ c ] + " ";
}