It's only common in romance, where a hiccup in publishing schedules plus having an author with multiple series means that you can run into weird situations -- I think Grace Burrowes will have eight books out this year.

In SF, the last time I heard of a situation like this was Charlie Stross's 2004, in which _The Atrocity Archive_ and _The Family Trade_ and _Iron Sunrise_ all came off the presses.

So: awesome!