Do database engines update when the updated value is the same?
You should assume the UPDATE is always writing the new value, even when identical. Write the query to eliminate no-ops with an appropriate WHERE clause. Approach it as a large table update, ie. do it in batches and make it idempotent. Is the only safe approach that is not victimized by assumptions and implementation specific details.

Case in point if you test prior to SQL Server 2012 you may conclude that an update that generates an identical in-place row image does not update the page and generates no log, while SQL 2012 under identical conditions you may conclude that it does not update the page but does generate log.