While winning an award is an impressive feat, there’s not particularly any reason to send this letter other than being a kind individual and showing appreciation. It’s a nice way to show that your business isn’t in the form of jealousy and that you can deal with being on the losing ends
Congratulate the individual on winning the award.
If you want and if it’s in your style, write some humor to play down the event.
Flatter the individual and mention how much the award is worth in terms of honor.
I would like to congratulate you for winning the New Employee award. This is your second year in a row that you’ve won the award and the employees in the office couldn’t be happier for you! It’s almost as if you’re stealing my ideas and presenting them as your own! I’m kidding of course, but congratulations on the award, as you’re the only employee to win that award twice in a row.