When I try to build an unique building (like a Palace) if you click quick enough when you add them to a city, it's possible to build two or three. The palace will go on the wait list, and finaly they will all be build.
This has happened to me on-and-off-again with every type of building, even things with 100% bonuses! At first I thought it was on purpose, like if you focus construction with the same building you can replicate it, but then I realized this was not the case.
I can confirm this, happend on and off in my games.