"yet" has about six meanings, and in the context we're dealing with it's ambiguous. It's synonymous with both once and still. The latter synonym, still means, not moving, and given our context, "extending congratulations yet/still again" implies there was a first (or even second) attempt at extending congratulations that wasn't successful. That's why "once", meaning one more time, would be a better choice for that particular context.
- For Teachers