Free buildings that I already have?
For Legalism, if a city would get a free culture building but already has built all researched ones, it will get the next culture building once it is researched.
I was curious about this, so today I started a game, built Monument in my capital, and took Legalism. The Monument still cost maintenance, but as soon as I was able to build Amphitheater, a free one popped up in my capital.
They do change into no maintenance version.