Is it common to refer to someone as "Prof. Dr. [LastName]"? I have generally only heard people refer to professors as either "Dr. Smith" or "Prof. Smith". However, I received an email that referred to the keynote speakers at a conference as "Prof. Dr. Smith". Is this common? (It is for a conference in taking place in the EU and I am from the US if that helps).
German academia traditionally expects that one will use all relevant titles, so Prof. Dr. is pretty common there. Likewise other places with an academic system related to Germany in some way. As you note, in the US this would be very uncommon and the two titles you mention often used interchangeably there, even when it isn't clear that both apply. 

And, if I remember correctly, it is always Prof. Dr. and never Dr. Prof. since the professorship was earned after the doctorate and is a "higher level" honorific. But "Herr Prof. Dr." doesn't fit that rule, I guess.